it has been assigned the constant
it has not been set to any value yet.
it has been unset().
<?php $var = NULL; ?>
This feature has been DEPRECATED as of PHP 7.2.0. Relying on this feature is highly discouraged.
The special NULL value represents a variable with no value. NULL is the only possible value of type null. The special null value does not belong to any type of those described above.
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.
The type of a variable is not usually set by the programmer; rather, it is decided at runtime by PHP depending on the context in which that variable is used. To forcibly convert a variable to a certain type, either cast the variable or use the settype() function on it. Note that a variable may be evaluated with different values in certain situations, depending on what type it is at the time.
Is a widely-used open source general-purpose scripting language that is especially suited for web development and can be embedded into HTML