Implement explicit send-by-ref

[iluuu1994]

Currently, in order to pass an argument by reference, the reference has to be declared at the definition-site, but not at the call-site:

function byRef(&$ref) {}
byRef($var);

This change adds the ability to specify the by reference pass at both the definition-site and the call-site:

function byRef(&$ref) {}
byRef(&$var);

Importantly, specifying a reference during the call will generate an error if the argument is not also declared by-reference at the declaration site:

function byVal($val) {}
byVal(&$val); // ERROR

This makes it different from the call-time-pass-by-reference feature in PHP 4. If this feature is used, the reference has to be declared at both the call- and declaration-site.

Note that it is still possible to not explicitly specify the use of by-reference passing at the call-site. As such, the following code remain legal:

function byRef(&$ref) {}
byRef($var);