Variables in PHP are identifiers prefixed with a dollar sign ($). For example:


A variable may hold a value of any type. There is no compile-time or runtime type checking on variables. You can replace a variable’s value with another of a different type:

$what = "Fred";
$what = 35;
$what = array("Fred", 35, "Wilma");

There is no explicit syntax for declaring variables in PHP. The first time the value of a variable is set, the variable is created. In other words, setting a value to a variable also functions as a declaration. For example, this is a valid complete PHP program:

$day = 60 * 60 * 24;
echo "There are {$day} seconds in a day.\n";

There are 86400 seconds in a day.

A variable whose value has not been set behaves like the NULL value:

if ($uninitializedVariable === NULL) {
  echo "Yes!";


Variable Variables

You can reference the value of a variable whose name is stored in another variable by prefacing the variable reference with an additional dollar sign ($). For example:

$foo = "bar";
$$foo = "baz";

After the second statement executes, the variable $bar has the value "baz".

Variable References

In PHP, references are how you create variable aliases. To make $black an alias for the variable $white, use:

$black =& $white;

The old value of $black, if any, is lost. Instead, $black is now another name for the value that is stored in $white:

$bigLongVariableName = "PHP";
$short =& $bigLongVariableName;
$bigLongVariableName .= " rocks!";

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