What is the significance of the error message "Passing null to parameter #1 ($haystack) of type string is deprecated" in PHP 8.1?

The error message "Passing null to parameter #1 ($haystack) of type string is deprecated" in PHP 8.1 indicates that passing a null value to a function or method that expects a string as its first parameter is no longer supported. To resolve this issue, you should ensure that you are passing a valid string value instead of null to the function or method.

// Before fix
$haystack = null;
$needle = 'needle';
$result = strpos($haystack, $needle);

// After fix
$haystack = '';
$needle = 'needle';
$result = strpos($haystack, $needle);