Php variables with variable names
Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.
Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$
Note: For our purposes here, a letter is a-z, A-Z, and the bytes from 128 through 255 ( 0x80-0xff ).
Note: $this is a special variable that can’t be assigned. Prior to PHP 7.1.0, indirect assignment (e.g. by using variable variables) was possible.
For information on variable related functions, see the Variable Functions Reference.
$var = ‘Bob’ ;
$Var = ‘Joe’ ;
echo » $var , $Var » ; // outputs «Bob, Joe»
$ 4site = ‘not yet’ ; // invalid; starts with a number
$_4site = ‘not yet’ ; // valid; starts with an underscore
$täyte = ‘mansikka’ ; // valid; ‘ä’ is (Extended) ASCII 228.
?>
By default, variables are always assigned by value. That is to say, when you assign an expression to a variable, the entire value of the original expression is copied into the destination variable. This means, for instance, that after assigning one variable’s value to another, changing one of those variables will have no effect on the other. For more information on this kind of assignment, see the chapter on Expressions.
PHP also offers another way to assign values to variables: assign by reference. This means that the new variable simply references (in other words, «becomes an alias for» or «points to») the original variable. Changes to the new variable affect the original, and vice versa.
To assign by reference, simply prepend an ampersand (&) to the beginning of the variable which is being assigned (the source variable). For instance, the following code snippet outputs ‘ My name is Bob ‘ twice:
$foo = ‘Bob’ ; // Assign the value ‘Bob’ to $foo
$bar = & $foo ; // Reference $foo via $bar.
$bar = «My name is $bar » ; // Alter $bar.
echo $bar ;
echo $foo ; // $foo is altered too.
?>
One important thing to note is that only named variables may be assigned by reference.
$foo = 25 ;
$bar = & $foo ; // This is a valid assignment.
$bar = &( 24 * 7 ); // Invalid; references an unnamed expression.
It is not necessary to initialize variables in PHP however it is a very good practice. Uninitialized variables have a default value of their type depending on the context in which they are used — booleans default to false , integers and floats default to zero, strings (e.g. used in echo ) are set as an empty string and arrays become to an empty array.
Example #1 Default values of uninitialized variables
// Unset AND unreferenced (no use context) variable; outputs NULL
var_dump ( $unset_var );
// Boolean usage; outputs ‘false’ (See ternary operators for more on this syntax)
echo $unset_bool ? «true\n» : «false\n» ;
// String usage; outputs ‘string(3) «abc»‘
$unset_str .= ‘abc’ ;
var_dump ( $unset_str );
// Integer usage; outputs ‘int(25)’
$unset_int += 25 ; // 0 + 25 => 25
var_dump ( $unset_int );
// Float usage; outputs ‘float(1.25)’
$unset_float += 1.25 ;
var_dump ( $unset_float );
// Array usage; outputs array(1) < [3]=>string(3) «def» >
$unset_arr [ 3 ] = «def» ; // array() + array(3 => «def») => array(3 => «def»)
var_dump ( $unset_arr );
// Object usage; creates new stdClass object (see http://www.php.net/manual/en/reserved.classes.php)
// Outputs: object(stdClass)#1 (1) < ["foo"]=>string(3) «bar» >
$unset_obj -> foo = ‘bar’ ;
var_dump ( $unset_obj );
?>
Relying on the default value of an uninitialized variable is problematic in the case of including one file into another which uses the same variable name. E_WARNING (prior to PHP 8.0.0, E_NOTICE ) level error is issued in case of working with uninitialized variables, however not in the case of appending elements to the uninitialized array. isset() language construct can be used to detect if a variable has been already initialized.
User Contributed Notes 5 notes
This page should include a note on variable lifecycle:
Before a variable is used, it has no existence. It is unset. It is possible to check if a variable doesn’t exist by using isset(). This returns true provided the variable exists and isn’t set to null. With the exception of null, the value a variable holds plays no part in determining whether a variable is set.
Setting an existing variable to null is a way of unsetting a variable. Another way is variables may be destroyed by using the unset() construct.
print isset( $a ); // $a is not set. Prints false. (Or more accurately prints ».)
$b = 0 ; // isset($b) returns true (or more accurately ‘1’)
$c = array(); // isset($c) returns true
$b = null ; // Now isset($b) returns false;
unset( $c ); // Now isset($c) returns false;
?>
is_null() is an equivalent test to checking that isset() is false.
The first time that a variable is used in a scope, it’s automatically created. After this isset is true. At the point at which it is created it also receives a type according to the context.
$a_bool = true ; // a boolean
$a_str = ‘foo’ ; // a string
?>
If it is used without having been given a value then it is uninitalized and it receives the default value for the type. The default values are the _empty_ values. E.g Booleans default to FALSE, integers and floats default to zero, strings to the empty string », arrays to the empty array.
A variable can be tested for emptiness using empty();
$a = 0 ; //This isset, but is empty
?>
Unset variables are also empty.
empty( $vessel ); // returns true. Also $vessel is unset.
?>
Everything above applies to array elements too.
$item = array();
//Now isset($item) returns true. But isset($item[‘unicorn’]) is false.
//empty($item) is true, and so is empty($item[‘unicorn’]
$item [ ‘unicorn’ ] = » ;
//Now isset($item[‘unicorn’]) is true. And empty($item) is false.
//But empty($item[‘unicorn’]) is still true;
$item [ ‘unicorn’ ] = ‘Pink unicorn’ ;
//isset($item[‘unicorn’]) is still true. And empty($item) is still false.
//But now empty($item[‘unicorn’]) is false;
?>
For arrays, this is important because accessing a non-existent array item can trigger errors; you may want to test arrays and array items for existence with isset before using them.
Php variables with variable names
It may be worth specifically noting, if variable names follow some kind of «template,» they can be referenced like this:
// Given these variables .
$nameTypes = array( «first» , «last» , «company» );
$name_first = «John» ;
$name_last = «Doe» ;
$name_company = «PHP.net» ;
// Then this loop is .
foreach( $nameTypes as $type )
print $ < "name_ $type " >. «\n» ;
// . equivalent to this print statement.
print » $name_first \n $name_last \n $name_company \n» ;
?>
This is apparent from the notes others have left, but is not explicitly stated.
In addition, it is possible to use associative array to secure name of variables available to be used within a function (or class / not tested).
This way the variable variable feature is useful to validate variables; define, output and manage only within the function that receives as parameter
an associative array :
array(‘index’=>’value’,’index’=>’value’);
index = reference to variable to be used within function
value = name of the variable to be used within function
$vars = [ ‘id’ => ‘user_id’ , ’email’ => ‘user_email’ ];
function validateVarsFunction ( $vars )
//$vars[‘id’]=34; // define allowed variables
$user_id = 21 ;
$user_email = ’email@mail.com’ ;
echo $vars [ ‘id’ ]; // prints name of variable: user_id
echo $< $vars [ 'id' ]>; // prints 21
echo ‘Email: ‘ .$< $vars [ 'email' ]>; // print email@mail.com
// we don’t have the name of the variables before declaring them inside the function
>
?>
The feature of variable variable names is welcome, but it should be avoided when possible. Modern IDE software fails to interpret such variables correctly, regular find/replace also fails. It’s a kind of magic 🙂 This may really make it hard to refactor code. Imagine you want to rename variable $username to $userName and try to find all occurrences of $username in code by checking «$userName». You may easily omit:
$a = ‘username’;
echo $$a;
If you want to use a variable value in part of the name of a variable variable (not the whole name itself), you can do like the following:
$price_for_monday = 10 ;
$price_for_tuesday = 20 ;
$price_for_wednesday = 30 ;
$price_for_today = $< 'price_for_' . $today >;
echo $price_for_today ; // will return 20
?>
PHP actually supports invoking a new instance of a class using a variable class name since at least version 5.2
class Foo public function hello () echo ‘Hello world!’ ;
>
>
$my_foo = ‘Foo’ ;
$a = new $my_foo ();
$a -> hello (); //prints ‘Hello world!’
?>
Additionally, you can access static methods and properties using variable class names, but only since PHP 5.3
class Foo public static function hello () echo ‘Hello world!’ ;
>
>
$my_foo = ‘Foo’ ;
$my_foo :: hello (); //prints ‘Hello world!’
?>
You may think of using variable variables to dynamically generate variables from an array, by doing something similar to: —
foreach ( $array as $key => $value )
$ $key = $value ;
>
?>
This however would be reinventing the wheel when you can simply use:
extract ( $array , EXTR_OVERWRITE );
?>
Note that this will overwrite the contents of variables that already exist.
Extract has useful functionality to prevent this, or you may group the variables by using prefixes too, so you could use: —
$array =array( «one» => «First Value» ,
«two» => «2nd Value» ,
«three» => «8»
);
extract ( $array , EXTR_PREFIX_ALL , «my_prefix_» );
?>
This would create variables: —
$my_prefix_one
$my_prefix_two
$my_prefix_three
containing: —
«First Value», «2nd Value» and «8» respectively
Another use for this feature in PHP is dynamic parsing..
Due to the rather odd structure of an input string I am currently parsing, I must have a reference for each particular object instantiation in the order which they were created. In addition, because of the syntax of the input string, elements of the previous object creation are required for the current one.
Normally, you won’t need something this convolute. In this example, I needed to load an array with dynamically named objects — (yes, this has some basic Object Oriented programming, please bare with me..)
// this is only a skeletal example, of course.
$object_array = array();
// assume the $input array has tokens for parsing.
foreach ( $input_array as $key => $value ) <
// test to ensure the $value is what we need.
$obj = «obj» . $key ;
$ $obj = new Obj ( $value , $other_var );
Array_Push ( $object_array , $ $obj );
// etc..
>
?>
Now, we can use basic array manipulation to get these objects out in the particular order we need, and the objects no longer are dependant on the previous ones.
I haven’t fully tested the implimentation of the objects. The scope of a variable-variable’s object attributes (get all that?) is a little tough to crack. Regardless, this is another example of the manner in which the var-vars can be used with precision where tedious, extra hard-coding is the only alternative.
Then, we can easily pull everything back out again using a basic array function: foreach.
//.
foreach( $array as $key => $object )
echo $key . » — » . $object -> print_fcn (). »
\n» ;
?>
Through this, we can pull a dynamically named object out of the array it was stored in without actually knowing its name.