“Call to undefined function get_magic_quotes_gpc()” in WordPress: the fix

The message, exactly

An old contact-form plugin checks for magic quotes before cleaning a message. On PHP 7.4 that gives a deprecation notice; on PHP 8.0 the page stops:

<?php
$message = 'It\'s ready';
if (get_magic_quotes_gpc()) {
    $message = stripslashes($message);
}
echo $message, "\n";

Deprecated: Function get_magic_quotes_gpc() is deprecated in /var/www/html/wp-content/plugins/old-contact/old-contact.php on line 3
It's ready

Fatal error: Uncaught Error: Call to undefined function get_magic_quotes_gpc() in /var/www/html/wp-content/plugins/old-contact/old-contact.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/wp-content/plugins/old-contact/old-contact.php on line 3

php.net’s page for the function: “This function has been DEPRECATED as of PHP 7.4.0, and REMOVED as of PHP 8.0.0.” (php.net)

Why removing the check is safe

“Magic quotes” was an old PHP setting that added a backslash before every quote in form data. PHP switched it off long before 7.4, and get_magic_quotes_gpc() only reported its state. For PHP 7.4, php.net says the functions “are deprecated. They always return false.” (php.net)

So in the example, stripslashes() never ran on PHP 7.4: the message came out the same with or without the check. Removing the check keeps exactly that behaviour; the fixed version below gives the same output on PHP 7.4, 8.0 and 8.5.

Fix it, in order

Found before you switch

The call is in the code itself, so a check that reads code finds it while your site still runs PHP 7.4:

Old code that calls it often has other PHP 8.0 removals nearby, such as each() or create_function().