Fixed php.Boot._hx_string_call error.

- Fixed passing arguments in wrong order calling `_hx_last_index_of`
  in `php.Boot._hx_string_call`.

This code can be used to test the error:

```haxe

package test;

import haxe.unit.TestCase;

class TestString extends TestCase {
	
	public function testString():Void {
		
		var str:String = 'Hello World!!';
		assertEquals(7, str.lastIndexOf('o')); // Works.
		
	}
	
	public function testDynamicString():Void {
		
		var str:Dynamic = 'Hello World!!';
		assertEquals(7, str.lastIndexOf('o')); // Fails.
		
	}
	
}

```