Changeset 55990

Code Modernization: Replace usage of substr() with str_starts_with() and str_ends_with().

str_starts_with() and str_ends_with() were introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) begins or ends with the given substring (needle).

WordPress core includes a polyfill for these functions on PHP < 8.0 as of WordPress 5.9.

This commit uses str_starts_with() and str_ends_with() in core files where appropriate:

• $needle === substr( $string, 0, $length ), where $length is the length of $needle, is replaced with str_starts_with( $haystack, $needle ).
• $needle === substr( $string, $offset ), where $offset is negative and the absolute value of $offset is the length of $needle, is replaced with str_ends_with( $haystack, $needle ).

This aims to make the code more readable and consistent, as well as better aligned with modern development practices.

Follow-up to [52039], [52040], [52326], [55703], [55710], [55987], [55988].

Props Soean, spacedmonkey, Clorith, ocean90, azaozz, sabernhardt, SergeyBiryukov.
Fixes #58220.