wp-includes/functions.php:1591Determines whether input is yes or no.
$ynstringboolfunction bool_from_yn( $yn ) { return ( 'y' === strtolower( $yn ) );}Introduced in 1.0.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/functions.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.