Can somebody explain to me why this codes returns "TRUE".
I know that i should use the "===" rather "==" but I run to this code and wondering why it returns to true. Thanks in advance.
<?php
$s = "final";
$i = 0;
if($s == $i){
echo "TRUE";
}else{
echo "FALSE";
}
When you are trying to compare string and number, interpretator converts your string to int, so you got 0 == 0 at final. Thats why string == 0 is true.
Take a look at the PHP comparison tables.
You can see in the "Loose comparisons with ==" table that comparing the number 0 with a string containing text ("php" in the example) evaluates to TRUE.
This is just a property of the loose comparisons implemented in PHP. I wouldn't search for any more logic behind this than that this is a given.
As mentionned above, it is an issue with php's loose comparison. The accepted answer on php string comparasion to 0 integer returns true? Explains it well enough IMHO. in short "==" attempts to cast your string into an int, and since it fails, the resulting int has a value of 0
From PHP comparison operators:
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.
And from PHP string conversion to numbers:
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).
So when you compare integer and a string, PHP tries to convert string to integer first and as "final" doesn't contain any valid numeric data, it is converted to 0.
You can try:
var_dump( intval('12final') ); //int(12)
var_dump( floatval('1.2final') ); //float(1.2)
This is because of both 12final and 1.2final start with valid numeric data (12 and 1.2 respecrively), their converted value is not 0.
Related
I encountered a strange and unexpected behavior in PHP while comparing some string values. The first two statements below return true when I would expect them to return false. The last statement returns false as expected. I'm aware of PHP's Type Juggling, but what I understand from the docs is that type juggling happens when you are comparing two different data types like a string and an integer. In the examples below though both literals are strings. Does this mean that when you are doing string comparison in PHP it inspects both strings to see if they look like integers and if so type casts the both of them to integers and then compares those integer values. So my question is under what conditions does this behavior happen, how exactly does string comparison work in PHP?
var_dump("10" == "10.0000");
var_dump("10" == "+10.");
var_dump("10" == "10 ");
#output
bool(true)
bool(true)
bool(false)
Updates
So baba's answer below comparison involves numerical strings really helped in getting me to understand what's going on. The function is_numeric will return to you whether or not a string is considered to be a numeric string. interestingly "10 " is not considered a numeric string but " 10" is. I dug around the PHP source code and I believe the implementation of is_numeric is in the is_numeric_string_ex function. From that one can tell exactly when PHP will treat a string as a numeric string.
You are getting error because of the position of the space this would return true
var_dump("10" == " 10"); // true
So if you RUN
var_dump("10" == "10 "); //false
What you are actually Running is because it would be treated as a string
var_dump("10" == 0); //false
This is because Type juggling would convert "10 " to 0 this is in the PHP Documentation
FROM PHP DOC
TRUE if $a is equal to $b after type juggling.
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. These rules also apply to the switch statement. The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.
If you want to trick type juggling
var_dump("10" == 0 + "10 "); // true
This is Because
An example of PHP's automatic type conversion is the addition operator '+'. If either operand is a float, then both operands are evaluated as floats, and the result will be a float. Otherwise, the operands will be interpreted as integers, and the result will also be an integer. Note that this does not change the types of the operands themselves; the only change is in how the operands are evaluated and what the type of the expression itself is.
I suppose the definite answer lies buried somewhere in the vastness of
compare_function
in php-src/Zend/zend_operators.c and the macros used in there.
In this case, all the 'strings' except the last one, are treated like floats or integers, and then compared. Thats why line 1 and 2 give a true.
In line 3 there is a space in the string, and that means for php that is is a 'real' string, and that line gets strings compared.
I do not like loose typed languages either, but for php this is the way it works.
if ("false" == 0) echo "true\n";
// => true
While PHP documentation says:
var_dump((bool) "false"); // bool(true)
If it is a bug where can I file it?
Nope, not a bug. Don't bother filing it.
This seemingly inconsistent behavior has nothing to do with the word "false". It has to do with your string consisting of only letters and no numeric digits, so when it gets cast to an integer for comparison (== 0), it results in 0, making 0 == 0 which evaluates to true.
If you compare any other alphabetic string to 0, you'll get the same result:
"abcd" == 0
"a" == 0
"true" == 0
It is not a bug, it's a well documented feature when comparing values of different datatypes - in this case, string to integer. PHP converts the string to an integer to do the comparison, and "false" converts to 0.
See the manual:
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.
and this page on string to number conversion
If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero).
To avoid this behaviour, use comparison with strict typing:
if ("false" === 0) echo "true\n";
The problem is that PHP casts variables unless you use a strict comparison. As others mentioned, the string "false" is being cast to an integer.
If you want to do a comparison without typecasting use:
"false" === 0
This might help: These are the only things that PHP considers false:
Actual boolean false
NULL values (empty)
The Integer 0
Try to avoid string comparisons of text and stick with booleans!
Anyone know why in php
var_dump( "cat" == 0 );
Evaluates to true? Also I realize:
var_dump( "cat" === 0 );
has the intended result, but curious as to why the first case would be true. Also this is php 5.3.
The string is being implicitly converted to an integer. See the documentation:
If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero).
The PHP Manual has a type comparison table in it, which gives you an idea of what happens when comparing variables of two different data types.
Your first example (a 'loose' comparison since it does not also check the data types of the two operands) implicitly converts the string on the left to an integer. Since it does not start with a number, the string is converted to the integer 0, which is equal to the integer 0.
Your second example compares not only the values but the types as well. Since the type is different, the comparison is false.
Because you are implicitly casting the string to an integer, and as the string does not contain numbers it is cast to a 0
"cat" is juggling to 0 as an integer, that is why it is true
but if you typed
var_dump( "01" == 0 );
it would have been false because 1 is not equals 0
$i=0;
$j='00:15';
if ($i==$j)
echo "equal";
else
echo "not equal";
displays equal?
00:15 is cast into an integer first and then compared with $i. Since intval() (which does the casting) stops at the first non-numeric character, 00:15 is cast into zero, which is equal to zero. No bug here, this is how the == operator works.
If you want to avoid casting, use === instead.
Using the == the types will be converted before they are compared. In this case, both will be converted to an integer, and when you convert a non-numeric string to an integer, it will probably result in 0.
If you use the === operator instead, you will get the result that you expect.
string and int compares as int's, So $j converts to 0 here because starts with 00
Not bizarre at all - when comparing string with integer, the first one is being cast to integer then compared. Use === for type checking and you'll see the real result
Turns out the === is problematic
for example 10===10 will be false if one is integer and one is string even though they are identical as can happen from forms posting.
the solution i used was:
if ("$i"=="$j")
this way it just checks if they're identical regardless of type
Whey does this evaluate to true?
<?php
$val2=0;
//outputs that is an error123
if($val2=='error123'){
echo 'that is an error123<br />';
}else{
echo 'that is not an error123<br />';
}
You're comparing a string to an integer. To make the comparison the string is first converted to an integer. When 'error123' is converted to an integer it becomes 0.
echo intval("error123");
Result:
0
In the PHP manual there is an explanation for this behaviour.
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.
There is a quick reference page PHP type comparison tables that shows you the result of various comparions. See the table "Loose comparisons with ==". The interesting part with regard to this question is that 0 == "php" is shown as TRUE.
There is also a page on type juggling. A user comment on that page gives nearly the exact same example as this.
If you don't want the type juggling use === instead of ==.
Give this a try: $val2==='error123'
That will evaluate the value and the type of the variable. More here:
http://us.php.net/manual/en/language.operators.comparison.php