`str_starts_with()` was introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) begins with the given substring (needle).
WordPress core includes a polyfill for `str_starts_with()` on PHP < 8.0 as of WordPress 5.9.
This commit replaces `0 === strpos( ... )` with `str_starts_with()` in core files, making the code more readable and consistent, as well as improving performance.
While `strpos()` is slightly faster than the polyfill on PHP < 8.0, `str_starts_with()` is noticeably faster on PHP 8.0+, as it is optimized to avoid unnecessarily searching along the whole haystack if it does not find the needle.
Follow-up to [52039], [52040], [52326].
Props spacedmonkey, costdev, sabernhardt, mukesh27, desrosj, jorbin, TobiasBg, ayeshrajans, lgadzhev, SergeyBiryukov.
Fixes#58012.
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