wp-includes/deprecated.php:2879Write contents to the file used for debugging.
$fpmixed$messagestringfunction debug_fwrite( $fp, $message ) { _deprecated_function( __FUNCTION__, '3.4.0', 'error_log()' ); if ( ! empty( $GLOBALS['debug'] ) ) error_log( $message );}Introduced in 0.71. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/deprecated.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.