Basics
Variables in PHP are represented by a dollar sign followed by the
name of the variable. The variable name is case-sensitive.
A valid variable name starts with a letter
(A-Z, a-z, or the bytes from 128 through 255)
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:
PHP doesn't support Unicode variable names, however, some character
encodings (such as UTF-8) encode characters in such a way that all bytes
of a multi-byte character fall within the allowed range, thus making it a
valid variable name.
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.
Example #1 Valid and invalid variable names
<?php
$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.
?>
PHP accepts a sequence of any bytes as a variable name. Variable
names that do not follow the above-mentioned naming rules can only be
accessed dynamically at runtime. See
variable variables
for information on how to access them.
Example #2 Accessing obscure variable names
<?php
${'invalid-name'} = 'bar';
$name = 'invalid-name';
echo ${'invalid-name'}, " ", $$name;
?>
The above example will output:
By default, variables are always assigned by value. That is to say,
when an expression is assigned 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:
One important thing to note is that only variables may be
assigned by reference.
It is not necessary to declare variables in PHP, however, it is a very
good practice. Accessing an undefined variable will result in an
E_WARNING (prior to PHP 8.0.0, E_NOTICE).
An undefined variable has a default value of null.
The isset() language
construct can be used to detect if a variable has already been initialized.
Example #3 Default value of an uninitialized variable
<?php
// Unset AND unreferenced (no use context) variable.
var_dump($unset_var);
?>
The above example will output:
Warning: Undefined variable $unset_var in ...
NULL
PHP allows array autovivification (automatic creation of new arrays)
from an undefined variable.
Appending an element to an undefined variable will create a new array and
will not generate a warning.
Example #4 Autovivification of an array from an undefined variable
<?php
$unset_array[] = 'value'; // Does not generate a warning.
?>
Warning
Relying on the default value of an uninitialized variable is problematic
when including one file in another which uses the same
variable name.
A variable can be destroyed by using the unset()
language construct.
For information on variable-related functions, see the
Variable Functions Reference.
