wp-includes/class-json.php:965Returns part of a string, interpreting $start and $length as number of bytes.
$string$start$lengthoptionalfalseinteger function substr8( $string, $start, $length=false ) { _deprecated_function( __METHOD__, '5.3.0', 'The PHP native JSON extension' ); if ( $length === false ) { $length = $this->strlen8( $string ) - $start; } if ( $this->_mb_substr ) { return mb_substr( $string, $start, $length, "8bit" ); } return substr( $string, $start, $length ); }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-json.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.