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Reference Guide: What does this symbol mean in PHP? (PHP Syntax)
(24 answers)
Closed 6 years ago.
I frequently see $var->another_var, or $somevar=>yet_another, or even $third_var->another=>$fourth_var in various snippets of code.
Is there some super amazing info-graphic somewhere that clearly explains the various usages and what they mean, specifically within a PHP context?
(In my case, using Drupal, which uses LOTS of arrays, but probably useful in lots of other CMSs / frameworks.)
EDIT: I have since been informed about a catch-all page that has a very useful, encyclopedic list of various symbols and syntaxes. However, I believe one section NOT covered there is the mix-and-match combo of $var->element=>$anothervar.
Single arrow - T_OBJECT_OPERATOR
->
This is used for access to an object property and the value associated with that property.
$object->property='value'
I have a dog and his name is Captain
$dog->name='Captain';
Now I have access to properties of my dog. The property that we have set is name
$dogName=$dog->name;
echo $dogName;
Will output: Captain
I can also add other properties and their associated value to my object.
$dog->weight='57lbs';
Now my object has two properties associated with it, name and weight.
Double arrow - T_DOUBLE_ARROW
=>
As is stated in the documentation an array is just a map of comma separated keys and the values associated with the key. The double arrow is essentially an assignment operator that assigns, or associates, the value to a key.
$array = array("key" => "value");
Again using the dog example.
$dog = array("name" => "Captain", "weight" => "57lbs");
And we can access values in my dog array by the respective keys.
$fatDog = $dog["weight"];
echo $fatDog;
Will output: 57lbs
Combinations of single and double arrow
$object->property=>$value;
This combines object/property with key/values. If we break it down into it's constituents it can make things much more clear.
We know that $object->property will yield the value associated with the property. Lets start by associating that with a variable:
$valueAssociatedWithProperty = $object->property;
Using substitution into the original gives:
$valueAssociatedWithProperty => $value;
We have seen that before it is just the key/value of an array! Lets apply this to the dog example and see what comes out:
$dog->name="Captain";
$description="He is crazy";
$array = array($dog->name => $description);
// $array = array("Captain" => "He is crazy");
$whatIsCaptain = $array["Captain"];
echo $whatIsCaptain;
He is crazy
I hope this helps.
Also look HERE for all the references you could ever hope for!
$var->another_var is "property another_var of object referenced by $var".
$somevar=>yet_another is used in array definitions, like this: $arr = array($somevar => yet_another). It would define an associative property with key equal to the value of variable $somevar, and value equal to the constant yet_another.
$third_var->another=>$fourth_var can be rewritten so it becomes more clear:
array( /*key=*/ ($third_var->another) => /*value=*/ $fourth_var )`
Related
I found what appears to be an abusable bug in the PHP language. When creating variable variables a seemingly illegal variable can be declared and then handled as long as you keep accessing it as a variable variable.
Example:
${'0'} = 1; //I am an illegal variable called $0
${'0'}++; //I can be accessed and manipulated.
var_dump(${'0'}); //Output: int(2)
This behavior appears rather odd. It is briefly mentioned in the official documentation for SimpleXml as a way to create variable names that contain hyphens but my example shows it can be abused pretty badly nonetheless.
I would like to know how come this behavior is possible and is tolerated when the code is parsed?
Internally PHP stores variables (zend_array* symbol_table) in a same data structure that is used for arrays. This allows you to have variable names with the same constraints as array keys.
Eg. Zend API function zend_set_local_var sets a value to symbol table using zend_hash_update, which is a function used to manipulate PHP array types as well.
We can't however allow every character in variable names in PHP source code. That's because lexical analysis must distinguish labels from other tokens. Variable variables offer a workaround for this.
It's not a bug nor a way to abuse anything. That being said, the documentation states that all labels, including variables, must have a regex form [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*, so i wouldn't rely on having arbitrary characters in variable names.
What ${'0'} = 1; actually does, it sets value 1 to the current scope's symbol table at key 0.
You can get that particular table with get_defined_vars function.
Looking at the source code, we'll see that the function just copies the current symbol table and returns it to caller.
PHP extract function (src) actually checks that keys have a valid label format (by calling php_valid_var_name), so you can't generate variables with funky names using that.
Anyway, even though it's possible to create variables of any name using variable variable syntax (even a variable with no name ${''}), i think it's bad practice. Even worse if a library expects or enforces you to do so. Saying it's a workaround might be a bit overstatement actually. Maybe it should be considered as an implementation detail and an undocumented feature.
#Nadav I don't have full idea about that but have a little bit idea,
Remember that curly braces({}) literally mean "evaluate what's inside the curly braces" so, you can squeeze the variable creation like you did ${'0'} = 1
reffered it from this http://php.net/manual/en/language.variables.php#73373
PHP stores variable in the array form, if you want to check then use $GLOBALS(an array) is the super global variable which has all the reference of the variables present in the script, it stores variable name as key and value as variable value, suppose below case:
<?php
${'0'} = 1; // here php stores it as 0th index in the array
${'1'} = 2; // here php stores it as 1th index in the array
$b = "I am b"; // here php stores it as bth index in the array
echo "<pre>";
print_r($GLOBALS);
output:
Array
(
[_GET] => Array
(
)
[_POST] => Array
(
)
[_COOKIE] => Array
(
)
[_FILES] => Array
(
)
[GLOBALS] => Array
*RECURSION*
[0] => 1
[1] => 2
[b] => I am b
)
In certain other languages (AS3 for example), it has been noted that initializing a new array is faster if done like this var foo = [] rather than var foo = new Array() for reasons of object creation and instantiation. I wonder whether there are any equivalences in PHP?
class Foo {
private $arr = array(); // is there another / better way?
}
$myArray = [];
Creates empty array.
You can push values onto the array later, like so:
$myArray[] = "tree";
$myArray[] = "house";
$myArray[] = "dog";
At this point, $myArray contains "tree", "house" and "dog". Each of the above commands appends to the array, preserving the items that were already there.
Having come from other languages, this way of appending to an array seemed strange to me. I expected to have to do something like $myArray += "dog" or something... or maybe an "add()" method like Visual Basic collections have. But this direct append syntax certainly is short and convenient.
You actually have to use the unset() function to remove items:
unset($myArray[1]);
... would remove "house" from the array (arrays are zero-based).
unset($myArray);
... would destroy the entire array.
To be clear, the empty square brackets syntax for appending to an array is simply a way of telling PHP to assign the indexes to each value automatically, rather than YOU assigning the indexes. Under the covers, PHP is actually doing this:
$myArray[0] = "tree";
$myArray[1] = "house";
$myArray[2] = "dog";
You can assign indexes yourself if you want, and you can use any numbers you want. You can also assign index numbers to some items and not others. If you do that, PHP will fill in the missing index numbers, incrementing from the largest index number assigned as it goes.
So if you do this:
$myArray[10] = "tree";
$myArray[20] = "house";
$myArray[] = "dog";
... the item "dog" will be given an index number of 21. PHP does not do intelligent pattern matching for incremental index assignment, so it won't know that you might have wanted it to assign an index of 30 to "dog". You can use other functions to specify the increment pattern for an array. I won't go into that here, but its all in the PHP docs.
In ECMAScript implementations (for instance, ActionScript or JavaScript), Array() is a constructor function and [] is part of the array literal grammar. Both are optimized and executed in completely different ways, with the literal grammar not being dogged by the overhead of calling a function.
PHP, on the other hand, has language constructs that may look like functions but aren't treated as such. Even with PHP 5.4, which supports [] as an alternative, there is no difference in overhead because, as far as the compiler/parser is concerned, they are completely synonymous.
// Before 5.4, you could only write
$array = array(
"foo" => "bar",
"bar" => "foo",
);
// As of PHP 5.4, the following is synonymous with the above
$array = [
"foo" => "bar",
"bar" => "foo",
];
If you need to support older versions of PHP, use the former syntax. There's also an argument for readability but, being a long-time JS developer, the latter seems rather natural to me. I actually made the mistake of trying to initialise arrays using [] when I was first learning PHP.
This change to the language was originally proposed and rejected due to a majority vote against by core developers with the following reason:
This patch will not be accepted because slight majority of the core developers voted against. Though if you take a accumulated mean between core developers and userland votes seems to show the opposite it would be irresponsible to submit a patch witch is not supported or maintained in the long run.
However, it appears there was a change of heart leading up to 5.4, perhaps influenced by the implementations of support for popular databases like MongoDB (which use ECMAScript syntax).
Prior to PHP 5.4:
$myArray = array();
PHP 5.4 and higher
$myArray = [];
In PHP an array is an array; there is no primitive vs. object consideration, so there is no comparable optimization to be had.
What you're doing is 100% correct.
In terms of nice naming it's often done that private/protected properties are preceded with an underscore to make it obvious that they're not public. E.g. private $_arr = array() or public $arr = array()
Initializing a simple array :
<?php $array1=array(10,20,30,40,50); ?>
Initializing array within array :
<?php $array2=array(6,"santosh","rahul",array("x","y","z")); ?>
Source : Sorce for the code
There is no other way, so this is the best.
Edit: This answer is not valid since PHP 5.4 and higher.
Try this:
$arr = (array) null;
var_dump($arr);
// will print
// array(0) { }
Do not do this:
$arrTst = array( 'IdxKeyOne' => null, 'IdxKeyTwo' => null, 'IdxKeyThr' => null );
There's no such thing as "initializing" an array's index-keys with dummy/placeholder values. print_r gives:
Array (
[IdxKeyOne] =>
[IdxKeyTwo] =>
[IdxKeyThr] =>
)
where the elements exist, having defined keys but null-values. When using the array later, you would have to drop the dummy-row anyway.
This question already has answers here:
How do I access this object property with an illegal name?
(2 answers)
Closed 10 months ago.
I need an escape sequence for - or the minus sign for php. The object has a name-value pair where the name happens to be having - between 2 words.
I can't do this using \ the standard escape sequence (- isn't anyways documented).
I can store the name in a $myvariable which can be used but out of curiosity is it possible to do the following?
$myobject->myweird-name
This gives an Error because of -
This is what you need:
$myobject->{'myweird-name'};
If you need to look up an index, there are a couple of ways of doing it:
// use a variable
$prop = 'my-crazy-property';
$obj->$prop;
// use {}
$obj->{'my-crazy-property'};
// get_object_vars (better with a lot of crazy properties)
$vars = get_object_vars($obj);
$vars['my-crazy-property'];
// you can cast to an array directly
$arr = (array)$obj;
$arr['my-crazy-property'];
If you need to work within a string (which is not your best idea, you should be using manual concatenation where possible as it is faster and parsed strings is unnecessary), then you should use {} to basically escape the entire sequence:
$foo = new stdClass();
$foo->{"my-crazy-property"} = 1;
var_dump("my crazy property is {$foo->{"my-crazy-property"}}";
Since you mentioned that this is LinkedIn's API which, I believe, has the option of returning XML, it might be faster (and possibly cleaner/clearer) to use the XML method calls and not use the objects themselves. Food for thought.
I know that prepending a '&' to your PHP variable sets up a reference to the original variable instead of copying its value like so:
$original = 'apples';
$secondary = &$original;
$original = 'oranges';
echo $secondary; // 'oranges'
If it works this way, why not just use the original variable then?
Passing by reference is useful and necessary when passing a variable as a parameter to a function, expecting that variable to be modified without a copy being created in memory. Many of PHP's native array_*() functions operate on array references, for example.
This function, for example, receives an array reference and appends an element onto the original array. If this was done without the & reference, a new array copy would be created in scope of the function. It would then have to be returned and reassigned to be used.
function add_to_an_array(&$array)
{
// Append a value to the array
$array[] = 'another value';
}
$array = array('one', 'two', 'three');
add_to_an_array($array);
print_r($array);
Array
(
[0] => one
[1] => two
[2] => three
[3] => another value
)
$original = 'apples';
function foo($word) {
$word = 'oranges';
}
foo($original);
echo $original; // apples, because only local $word was changed, not $original.
foo(&$original);
echo $original; // oranges, because $original and $word are the same
Pass by reference is really a cop out and goes against good encapsulation. If you need to manipulate a variable in that way, it probably should belong to a class as a member variable and then does not need to be passed to the function. Good OO design would usually make member variables immutable with a "final" keyword, but PHP doesn't have this. It's not intuitive that passing a variable to a function might change it's value which is why it should be avoided in most cases.
Also going to a more full OO design prevents you have having method signatures that are long and complex with many optional parameters that are difficult to re-factor.
A more interesting use of the is when it's used in the formal argument to a function
foo($a);
...
function foo (&$a) {
....
}
this allows you to modify a in the function.
There are many uses for references.
You can pass a reference to a variable to a function so you can change the value inside the function
You can use references to create linked lists
etc...
Just keep in mind that they're there, and you'll surely find an application for them when the time comes and you face a problem that can be solved with references.
Check out the following article for other ideas and uses of references:
http://www.elated.com/articles/php-references/
In certain other languages (AS3 for example), it has been noted that initializing a new array is faster if done like this var foo = [] rather than var foo = new Array() for reasons of object creation and instantiation. I wonder whether there are any equivalences in PHP?
class Foo {
private $arr = array(); // is there another / better way?
}
$myArray = [];
Creates empty array.
You can push values onto the array later, like so:
$myArray[] = "tree";
$myArray[] = "house";
$myArray[] = "dog";
At this point, $myArray contains "tree", "house" and "dog". Each of the above commands appends to the array, preserving the items that were already there.
Having come from other languages, this way of appending to an array seemed strange to me. I expected to have to do something like $myArray += "dog" or something... or maybe an "add()" method like Visual Basic collections have. But this direct append syntax certainly is short and convenient.
You actually have to use the unset() function to remove items:
unset($myArray[1]);
... would remove "house" from the array (arrays are zero-based).
unset($myArray);
... would destroy the entire array.
To be clear, the empty square brackets syntax for appending to an array is simply a way of telling PHP to assign the indexes to each value automatically, rather than YOU assigning the indexes. Under the covers, PHP is actually doing this:
$myArray[0] = "tree";
$myArray[1] = "house";
$myArray[2] = "dog";
You can assign indexes yourself if you want, and you can use any numbers you want. You can also assign index numbers to some items and not others. If you do that, PHP will fill in the missing index numbers, incrementing from the largest index number assigned as it goes.
So if you do this:
$myArray[10] = "tree";
$myArray[20] = "house";
$myArray[] = "dog";
... the item "dog" will be given an index number of 21. PHP does not do intelligent pattern matching for incremental index assignment, so it won't know that you might have wanted it to assign an index of 30 to "dog". You can use other functions to specify the increment pattern for an array. I won't go into that here, but its all in the PHP docs.
In ECMAScript implementations (for instance, ActionScript or JavaScript), Array() is a constructor function and [] is part of the array literal grammar. Both are optimized and executed in completely different ways, with the literal grammar not being dogged by the overhead of calling a function.
PHP, on the other hand, has language constructs that may look like functions but aren't treated as such. Even with PHP 5.4, which supports [] as an alternative, there is no difference in overhead because, as far as the compiler/parser is concerned, they are completely synonymous.
// Before 5.4, you could only write
$array = array(
"foo" => "bar",
"bar" => "foo",
);
// As of PHP 5.4, the following is synonymous with the above
$array = [
"foo" => "bar",
"bar" => "foo",
];
If you need to support older versions of PHP, use the former syntax. There's also an argument for readability but, being a long-time JS developer, the latter seems rather natural to me. I actually made the mistake of trying to initialise arrays using [] when I was first learning PHP.
This change to the language was originally proposed and rejected due to a majority vote against by core developers with the following reason:
This patch will not be accepted because slight majority of the core developers voted against. Though if you take a accumulated mean between core developers and userland votes seems to show the opposite it would be irresponsible to submit a patch witch is not supported or maintained in the long run.
However, it appears there was a change of heart leading up to 5.4, perhaps influenced by the implementations of support for popular databases like MongoDB (which use ECMAScript syntax).
Prior to PHP 5.4:
$myArray = array();
PHP 5.4 and higher
$myArray = [];
In PHP an array is an array; there is no primitive vs. object consideration, so there is no comparable optimization to be had.
What you're doing is 100% correct.
In terms of nice naming it's often done that private/protected properties are preceded with an underscore to make it obvious that they're not public. E.g. private $_arr = array() or public $arr = array()
Initializing a simple array :
<?php $array1=array(10,20,30,40,50); ?>
Initializing array within array :
<?php $array2=array(6,"santosh","rahul",array("x","y","z")); ?>
Source : Sorce for the code
There is no other way, so this is the best.
Edit: This answer is not valid since PHP 5.4 and higher.
Try this:
$arr = (array) null;
var_dump($arr);
// will print
// array(0) { }
Do not do this:
$arrTst = array( 'IdxKeyOne' => null, 'IdxKeyTwo' => null, 'IdxKeyThr' => null );
There's no such thing as "initializing" an array's index-keys with dummy/placeholder values. print_r gives:
Array (
[IdxKeyOne] =>
[IdxKeyTwo] =>
[IdxKeyThr] =>
)
where the elements exist, having defined keys but null-values. When using the array later, you would have to drop the dummy-row anyway.