[php] Generate PHP type declarations

We now automatically generate PHP type declarations for PHP >= 8.0.

The generated code still compiles with PHP 7.x but without type declarations.

Examples/test-suite/php/li_factory_runme.php

@@ -17,4 +17,7 @@ $point = Geometry::create(Geometry::POINT);
 $w = $point->width();
 check::equal($w, 1.0, "w failed");
 
+$point = Geometry::create(Geometry::SHAPELESS);
+check::equal($point, NULL, "NULL failed");
+
 check::done();