wp-includes/SimplePie/src/Cache/File.php:137Retrieve the last modified time for the cache
int public function mtime() { return @filemtime($this->name); }One change between 6.7.7 and 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
int to int|false.verified against sourcesrc/wp-includes/SimplePie/src/Cache/File.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.