Description:
------------
Using is_callable with the syntax_only parameter set to true does actually *not* 
check the syntax, for example of a valid classname or FQCN.

Also not for the method name.

My feature request is, that it will always check those strings according to the 
rules set in the PHP parser of the same PHP version the function ships with so 
that it can be used to validate PHP syntax as well.

Same for calls with :: for static class name method calls.

Test script:
---------------
var_dump(is_callable(['', ''], true));
var_dump(is_callable(['', 'method'], true));
var_dump(is_callable(['0', 'method'], true));
var_dump(is_callable(['0\\foo', 'method'], true));
var_dump(is_callable(['\\0\\foo', 'method'], true));

Expected result:
----------------
bool(false)
bool(false)
bool(false)
bool(false)
bool(false)

Actual result:
--------------
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)