wp-includes/class-wp-user.php:277Magic method for checking the existence of a certain custom field.
$keystringbool public function __isset( $key ) { if ( 'id' === $key ) { _deprecated_argument( 'WP_User->id', '2.1.0', sprintf( /* translators: %s: WP_User->ID */ __( 'Use %s instead.' ), '<code>WP_User->ID</code>' ) ); $key = 'ID'; } if ( isset( $this->data->$key ) ) { return true; } if ( isset( self::$back_compat_keys[ $key ] ) ) { $key = self::$back_compat_keys[ $key ]; } return metadata_exists( 'user', $this->ID, $key ); }Introduced in 3.3.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-user.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.