Give more details when function is disabled in php.ini

Currently, if a function is disabled in `php.ini` file, the output is the following:

> [BadFunctionCallException]
> This function is referenced in the php.ini 'disable_functions' and can't be called.  

This PR changes the exception message to give the disabled function name:

> [BadFunctionCallException]
> Function 'pcntl_signal' is referenced in the php.ini 'disable_functions' and can't be called.