For the record, I know the solution is to use === instead of == .
I'm just wondering what the logic behind it is. How is it logical that 'hello' can equal TRUE?
$var = TRUE;
if($var == 'hello'){
echo 'match';
}
else{
echo 'no match';
}
The solution has been discussed, but I haven't seen any real explanation.
String value equals true
== compares just the values of the variables whereas === compares variable values and type. so for an example:
1 == 1: true
1 === "1": false // "1" is a string and 1 is an integer
when asking if a string == true, you are essientially asking if it is set. Similar functionality is behind the isset() method.
If you were to compare "hello" === true. This would be false as they are of different type and "hello" would HAVE to equal "hello"
When using == operator think in falsy and truthy terms.
So:
When converting to boolean, the following values are considered FALSE:
the boolean FALSE itself
the integers 0 and -0 (zero)
the floats 0.0 and -0.0 (zero)
the empty string, and the string "0"
an array with zero elements
the special type NULL (including unset variables)
SimpleXML objects created from empty tags
-1 is considered TRUE, like any other non-zero (whether negative or positive) number!
Every other value is considered TRUE (including any resource and NAN).
#see: https://www.php.net/manual/en/language.types.boolean.php
Related
For the record, I know the solution is to use === instead of == .
I'm just wondering what the logic behind it is. How is it logical that 'hello' can equal TRUE?
$var = TRUE;
if($var == 'hello'){
echo 'match';
}
else{
echo 'no match';
}
The solution has been discussed, but I haven't seen any real explanation.
String value equals true
== compares just the values of the variables whereas === compares variable values and type. so for an example:
1 == 1: true
1 === "1": false // "1" is a string and 1 is an integer
when asking if a string == true, you are essientially asking if it is set. Similar functionality is behind the isset() method.
If you were to compare "hello" === true. This would be false as they are of different type and "hello" would HAVE to equal "hello"
When using == operator think in falsy and truthy terms.
So:
When converting to boolean, the following values are considered FALSE:
the boolean FALSE itself
the integers 0 and -0 (zero)
the floats 0.0 and -0.0 (zero)
the empty string, and the string "0"
an array with zero elements
the special type NULL (including unset variables)
SimpleXML objects created from empty tags
-1 is considered TRUE, like any other non-zero (whether negative or positive) number!
Every other value is considered TRUE (including any resource and NAN).
#see: https://www.php.net/manual/en/language.types.boolean.php
I have the following piece of code:
$item['price'] = 0;
/* Code to get item information goes in here */
if($item['price'] == 'e') {
$item['price'] = -1;
}
It is intended to initialize the item price to 0 and then get information about it. If the price is informed as 'e' it means an exchange instead of a sell, which is stored in a database as a negative number.
There is also the possibility to leave the price as 0, either because the item is a bonus or because the price will be set in a later moment.
But, whenever the price is not set, which leaves it with the initial value of 0, the if loop indicated above evaluates as true and the price is set to -1. That is, it considers 0 as equal to 'e'.
How can this be explained?
When the price is provided as 0 (after initialization), the behavior is erratic: sometimes the if evaluates as true, sometimes it evaluates as false.*
You are doing == which sorts out the types for you.
0 is an int, so in this case it is going to cast 'e' to an int. Which is not parsable as one and will become 0. A string '0e' would become 0 and would match!
Use ===
From PHP.net:
Comparisons between strings and numbers using == and other non-strict
comparison operators currently work by casting the string to a number,
and subsequently performing a comparison on integers or floats. This
results in many surprising comparison results, the most notable of
which is that 0 == "foobar" returns true.
However this behavior was changed in PHP 8.0:
When comparing to a numeric string, PHP 8 uses a number comparison.
Otherwise, it converts the number to a string and uses a string
comparison.
PHP 7
0 == 'foobar' // true
0 == '' // true
4 == '4e' // true (4e is cast as a number and becomes 4)
PHP 8 converts numbers to strings before making comparisons
0 == 'foobar' // false
0 == '' // false
4 == '4e' // false ('4e' is considered non-numeric therefore 4 is cast as a string and becomes '4')
This is a major change therefore it was implemented in a new major PHP version. This change breaks backward compatibility in scripts that depend on the old behavior.
This is due to how PHP does the comparison operation that the == comparison operator denotes:
If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically. […] The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.
As the first operand is a number (0) and the second is a string ('e'), the string is also converted to a number (see also table Comparison with Various Types). The manual page on the string data type defined how the string to number conversion is done:
When a string is evaluated in a numeric context, the resulting value and type are determined as follows.
If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.
In this case the string is 'e' and thus it will be evaluated as a float:
The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits.
As 'e' does not start with a valid numeric data, it evaluates to float 0.
"ABC" == 0
evaluates true because first "ABC" is converted to integer and becomes 0 then it is compared to 0.
This is an odd behaviour of the PHP language: normally one would expect 0 to be promoted to string "0" and then compared to "ABC" with a result false.
Perhaps that's what happen in other languages like JavaScript where the weak comparison "ABC" == 0 evaluates false.
Doing a strict comparison solves the problem:
"ABC" === 0
evaluates false.
But what if I do need to compare numbers as strings with numbers?
"123" === 123
evaluates false because the left and right term are of different type.
What is actually needed is a weak comparison without the pitfalls of PHP type juggling.
The solution is to explicit promote the terms to string and then do a comparison (strict or weak doesn't matter anymore).
(string)"123" === (string)123
is
true
while
(string)"123" === (string)0
is
false
Applied to the original code:
$item['price'] = 0;
/*code to get item information goes in here*/
if((string)$item['price'] == 'e') {
$item['price'] = -1;
}
The == operator will try to match values even if they are of different types. For instance:
'0' == 0 will be true
If you need type comparison as well, use the === operator:
'0' === 0 will be false
Your problem is the double equal operator, which will typecast the right member to the type of the left. Use strict if you prefer.
if($item['price'] == 'e') {
$item['price'] = -1;
}
Let's go back to your code (copied above). In this case, in most cases, $item['price'] is an integer (except when it is equal to e, obviously). As such, by laws of PHP, PHP will typecast "e" to integer, which yields int(0). (Don't believe me? <?php $i="e"; echo (int)$i; ?>).
To easily get away from this, use the triple equal (exact comparison) operator, which will check the type and will not implicitly typecast.
P.S: a PHP fun fact: a == b does not imply that b == a. Take your example and reverse it: if ("e" == $item['price']) will never actually be fulfilled provided that $item['price'] is always an integer.
There's a rather handy method in PHP for validating a mix of "0", "false", "off" as == false and "1", "on", "true" as == true which is often overlooked. It's particularly useful for parsing GET/POST arguments:
filter_var( $item['price'], FILTER_VALIDATE_BOOLEAN );
It's not wholy relevant to this use-case but given the similarity and fact this is the result search tends to find when asking the question of validating (string)"0" as false I thought it would help others.
http://www.php.net/manual/en/filter.filters.validate.php
You should use === instead of ==, because the ordinary operator does not compare the types. Instead it will attempt to typecast the items.
Meanwhile the === takes in consideration type of items.
=== means "equals",
== means "eeeeh .. kinda looks like"
Basically, always use the === operator, to guarantee type safety.
I think it is best to show by examples I did, while running into the same weird behavior. See my test case and hopefully it will help you understand the behavior better:
// Normal comparison using the == Operator
echo (0 == "0"); // true
echo (0 == "a"); // true
echo (0 == "safta!"); // true
echo (1000 == "bla"); // false. It appears that PHP has a weird behavior only with the number / string 0 / "0" according to the past 3 examples.
echo (23 == "23"); // true. So as we said, PHP has a problem (not a problem but weird behavior) only when the number / string 0 (or "0") is present
echo (23 == "24"); // false. values aren't equal (unlike last example). The type is less relevant with the == operator as we can see.
// Now using the === and !== Operators
echo ("0" === 0); // false, since === requires both value and type to be the same. Here, type is different (int vs string)
echo ("0" !== 0); // true because they aren't the same in terms of === comparison (type is different and that's why it's true)
echo ("bla" === "blaa"); // false because the values are not the same. The type is the same, but === checks for both equal type and equal value.
//Now using casting and === Operator:
echo ((string)123 === "123"); // true. The casting of the int 123 to string changed it to "123" and now both variables have same value and are of same type
echo ((int)"123" === 123); // true. The casting of the string 123 to int, changed it to int, and now both variables are of same value and type (which is exactly what the === operator is looking for)
// Now using casting and == Operator. Basically, as we've seen above, the == care less for the
// type of var, but more to the value. So the casting is less relevant here, because even
// without casting, like we saw earlier, we can still compare string to int with the == operator
// and if their value is same, we'll get true. Either way, we will show that:
echo ((string)123 == "123"); // true. The casting of the int 123 to string changed it to "123" and now both vars have same value and are of same type
echo ((int)"123" == 123); // true. The casting of the string 123 to int, changed it to int, and now both vars are of same value and type (which is exactly what the === operator is looking for)
I have the following piece of code:
$item['price'] = 0;
/* Code to get item information goes in here */
if($item['price'] == 'e') {
$item['price'] = -1;
}
It is intended to initialize the item price to 0 and then get information about it. If the price is informed as 'e' it means an exchange instead of a sell, which is stored in a database as a negative number.
There is also the possibility to leave the price as 0, either because the item is a bonus or because the price will be set in a later moment.
But, whenever the price is not set, which leaves it with the initial value of 0, the if loop indicated above evaluates as true and the price is set to -1. That is, it considers 0 as equal to 'e'.
How can this be explained?
When the price is provided as 0 (after initialization), the behavior is erratic: sometimes the if evaluates as true, sometimes it evaluates as false.*
You are doing == which sorts out the types for you.
0 is an int, so in this case it is going to cast 'e' to an int. Which is not parsable as one and will become 0. A string '0e' would become 0 and would match!
Use ===
From PHP.net:
Comparisons between strings and numbers using == and other non-strict
comparison operators currently work by casting the string to a number,
and subsequently performing a comparison on integers or floats. This
results in many surprising comparison results, the most notable of
which is that 0 == "foobar" returns true.
However this behavior was changed in PHP 8.0:
When comparing to a numeric string, PHP 8 uses a number comparison.
Otherwise, it converts the number to a string and uses a string
comparison.
PHP 7
0 == 'foobar' // true
0 == '' // true
4 == '4e' // true (4e is cast as a number and becomes 4)
PHP 8 converts numbers to strings before making comparisons
0 == 'foobar' // false
0 == '' // false
4 == '4e' // false ('4e' is considered non-numeric therefore 4 is cast as a string and becomes '4')
This is a major change therefore it was implemented in a new major PHP version. This change breaks backward compatibility in scripts that depend on the old behavior.
This is due to how PHP does the comparison operation that the == comparison operator denotes:
If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically. […] The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.
As the first operand is a number (0) and the second is a string ('e'), the string is also converted to a number (see also table Comparison with Various Types). The manual page on the string data type defined how the string to number conversion is done:
When a string is evaluated in a numeric context, the resulting value and type are determined as follows.
If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.
In this case the string is 'e' and thus it will be evaluated as a float:
The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits.
As 'e' does not start with a valid numeric data, it evaluates to float 0.
"ABC" == 0
evaluates true because first "ABC" is converted to integer and becomes 0 then it is compared to 0.
This is an odd behaviour of the PHP language: normally one would expect 0 to be promoted to string "0" and then compared to "ABC" with a result false.
Perhaps that's what happen in other languages like JavaScript where the weak comparison "ABC" == 0 evaluates false.
Doing a strict comparison solves the problem:
"ABC" === 0
evaluates false.
But what if I do need to compare numbers as strings with numbers?
"123" === 123
evaluates false because the left and right term are of different type.
What is actually needed is a weak comparison without the pitfalls of PHP type juggling.
The solution is to explicit promote the terms to string and then do a comparison (strict or weak doesn't matter anymore).
(string)"123" === (string)123
is
true
while
(string)"123" === (string)0
is
false
Applied to the original code:
$item['price'] = 0;
/*code to get item information goes in here*/
if((string)$item['price'] == 'e') {
$item['price'] = -1;
}
The == operator will try to match values even if they are of different types. For instance:
'0' == 0 will be true
If you need type comparison as well, use the === operator:
'0' === 0 will be false
Your problem is the double equal operator, which will typecast the right member to the type of the left. Use strict if you prefer.
if($item['price'] == 'e') {
$item['price'] = -1;
}
Let's go back to your code (copied above). In this case, in most cases, $item['price'] is an integer (except when it is equal to e, obviously). As such, by laws of PHP, PHP will typecast "e" to integer, which yields int(0). (Don't believe me? <?php $i="e"; echo (int)$i; ?>).
To easily get away from this, use the triple equal (exact comparison) operator, which will check the type and will not implicitly typecast.
P.S: a PHP fun fact: a == b does not imply that b == a. Take your example and reverse it: if ("e" == $item['price']) will never actually be fulfilled provided that $item['price'] is always an integer.
There's a rather handy method in PHP for validating a mix of "0", "false", "off" as == false and "1", "on", "true" as == true which is often overlooked. It's particularly useful for parsing GET/POST arguments:
filter_var( $item['price'], FILTER_VALIDATE_BOOLEAN );
It's not wholy relevant to this use-case but given the similarity and fact this is the result search tends to find when asking the question of validating (string)"0" as false I thought it would help others.
http://www.php.net/manual/en/filter.filters.validate.php
You should use === instead of ==, because the ordinary operator does not compare the types. Instead it will attempt to typecast the items.
Meanwhile the === takes in consideration type of items.
=== means "equals",
== means "eeeeh .. kinda looks like"
Basically, always use the === operator, to guarantee type safety.
I think it is best to show by examples I did, while running into the same weird behavior. See my test case and hopefully it will help you understand the behavior better:
// Normal comparison using the == Operator
echo (0 == "0"); // true
echo (0 == "a"); // true
echo (0 == "safta!"); // true
echo (1000 == "bla"); // false. It appears that PHP has a weird behavior only with the number / string 0 / "0" according to the past 3 examples.
echo (23 == "23"); // true. So as we said, PHP has a problem (not a problem but weird behavior) only when the number / string 0 (or "0") is present
echo (23 == "24"); // false. values aren't equal (unlike last example). The type is less relevant with the == operator as we can see.
// Now using the === and !== Operators
echo ("0" === 0); // false, since === requires both value and type to be the same. Here, type is different (int vs string)
echo ("0" !== 0); // true because they aren't the same in terms of === comparison (type is different and that's why it's true)
echo ("bla" === "blaa"); // false because the values are not the same. The type is the same, but === checks for both equal type and equal value.
//Now using casting and === Operator:
echo ((string)123 === "123"); // true. The casting of the int 123 to string changed it to "123" and now both variables have same value and are of same type
echo ((int)"123" === 123); // true. The casting of the string 123 to int, changed it to int, and now both variables are of same value and type (which is exactly what the === operator is looking for)
// Now using casting and == Operator. Basically, as we've seen above, the == care less for the
// type of var, but more to the value. So the casting is less relevant here, because even
// without casting, like we saw earlier, we can still compare string to int with the == operator
// and if their value is same, we'll get true. Either way, we will show that:
echo ((string)123 == "123"); // true. The casting of the int 123 to string changed it to "123" and now both vars have same value and are of same type
echo ((int)"123" == 123); // true. The casting of the string 123 to int, changed it to int, and now both vars are of same value and type (which is exactly what the === operator is looking for)
I've been reading the PHP Docs on Type Juggling and Booleans but I still don't understand why this comparison evaluates as true. My [incorrect] understanding tells me that in the below if statement, the integer 0 is considered FALSE and "a", being a non-empty string is considered TRUE. Therefore, I expected this comparison to resolve to FALSE == TRUE and ultimately, FALSE. Which part did I get wrong?
<?php
if(0 == "a"){
$result = "TRUE";
}else{
$result = "FALSE";
}
//$result == "TRUE"
?>
http://codepad.viper-7.com/EjxBF5
When PHP does a string <=> integer comparison, it attempts to convert the string to a number in an intelligent way. The assumption is that if you have a string "42" then you want to compare the value 42 to the other integer. When the string doesn't start with numbers, then its value is zero.
From the docs:
The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero).
This behavior is also inferred in the comparison docs (look at the first line in the first example).
Your mistake is that you assume operator == coerces each of its operands to boolean before comparing them. It does no such thing.
What happens is that since you are comparing an integer to a string, the string is converted to an integer (in this case "a" converts to 0) and then the comparison 0 == 0 is performed.
It will work if you use a strict comparison === instead of ==. The strict comparison also checks the type of the variables, so 0 === 'a' would be false.
It is in this way:
$arr_val = array(0,1,'0','1');
foreach ($arr_val as $key){
echo ($key == "TEST")?"EQUALLED":"NOT EQUALLED"."<br>";
}
0 == "TEST" prints "EQUALLED"
1 == "TEST" prints "NOT EQUALLED"
'0' =="TEST" prints "NOT EQUALLED"
'1' =="TEST" prints "NOT EQUALLED"
When I say it prints the value "SELECTED". But why the above first case prints equalled. Any ideas on this please? How would this be equal to. We know the fix to do comparision with
(===) operator. But I am trying to know the reason why (0=="TEST") is true.
When you provide PHP's == operator with a mixture of numeric and string operands, PHP will attempt to convert the string to the matching numeric type, part of a process it calls "type juggling". In this case, the string "TEST" converts to integer 0 so your test is equivalent to 0 == 0 which is true.
PHP provides the === operator, for testing if the value and type of both operands are equal. So while 0 == "TEST" will evaulate to true, 0 === "TEST" will not, and neither will 0 === "0" or 0 === 0.0.
Note that when PHP converts a string to a number, it attempts to parse the string for a valid number. See intval for more information on how it does this. Had you'd written 0 == "1TEST", the expression would have evaulated to 0 == 1, or false.
In your second example, 1 == "TEST", the string "TEST" is again converted to an integer resulting in 1 == 0, which is false.
Your last two examples use string comparisons. There is no conversion involved, and the results are self-explanatory.
PHP provides a comprehensive breakdown of how variables of different type compare for equality.
Because 0 is an integer, behind the scenes, this is the comparison that happens:
0 == intval( "TEST" )