ForeignAPIRepo and mwEmbed make use of json_decode, but that's only available with PHP 5 >= 5.2.0 (or PECL json extension), conflicting with the requeriment of just PHP 5.
Those cases should use the implementation from include/api/ApiFormatJson_json.php if native json_decode isn't available.
Greg Sabino Mullani reports that not having json (default CentOS install) produces
PHP Fatal error: Call to undefined function json_decode() in /var/www/html/bucardo.org/mwp3/js2/mwEmbed/php/jsAutoloadLocalClasses.php on line 23
Version: 1.16.x
Severity: major