Question

Consider the following example:

// does not work
foo( func_num_args() );

// works
$args = func_num_args();
foo( $args );

Why specifically does the former fail and the latter work?

Answer 1

Note:

Because this function depends on the current scope to determine
parameter details, it cannot be used as a function parameter in
versions prior to 5.3.0. If this value must be passed, the results
should be assigned to a variable, and that variable should be passed.

http://php.net/func_num_args

In other words: because of the way this function works internally. The runtime is in a different state when it’s inside a regular function body and when it’s evaluating expressions to be used as parameters to another function. This was changed in 5.3.