Undefined Variables in PHP

Undefined variables are those that have not yet been initialized with a value prior to being read.

⚠️ If we use an undefined variable we receive a warning and the PHP script continues its execution. Example:

<?php
    $audience = "World";
    
    echo $message;
    echo $audience;
?>

After have ran the previous code, we’re going to receive this warning message:

PHP Warning:  Undefined variable $message in index.php on line 4

However PHP still will print the value of the variable $audience:

World

This behavior is going to change on PHP 9, where it will throw an error, according to this RFC PHP: rfc:undefined_variable_error_promotion.

References

PHP: Basics - Manual