Like Zack said in the comments below you are able to simply re-instantiate it using
$foo = array(); // $foo is still here
If you want something more powerful use unset since it also will clear $foo from the symbol table, if you need the array later on just instantiate it again.
unset($foo); // $foo is gone
$foo = array(); // $foo is here again
If we are talking about very large tables I’d probably recommend
$foo = null;
unset($foo);
since that also would clear the memory a bit better. That behavior (GC) is however not very constant and may change over PHP versions. Bear in mind that re-instantiating a structure is not the same as emptying it.