'callable' was implemented as a typehint in 5.4
PHP 5.4.0 offers a wide range of new features:
As of PHP 5.4, the CLI (using readline) no longer dies on fatal errors (for example calling undefined functions).
LDAP supports paged results. See http://php.net/manual/en/function.ldap-control-paged-result.php for details.
The keyword 'insteadof' is introduced with PHP 5.4 http://php.net/language.oop5.traits.php