为什么回调函数在array_filter()里面没有起作用？

	<?php		function myFunction($v) 		{		if ($v==="Dog")			{				return "Fido";			}				return $v;		}		$a=array("Horse","Dog","Cat");		//Array ( [0] => Horse [1] => Fido [2] => Cat ) 		print_r(array_map("myFunction",$a)); 				//Array ( [0] => Horse [1] => Dog [2] => Cat )		//为什么回调函数在array_filter()里面没有起作用？		print_r(array_filter($a,"myFunction"));	 ?>

function myFunction($v)     {    if ($v==="Dog")        {            return true;        }            return false;    }    $a=array("Horse","Dog","Cat");    //Array ( [0] => Horse [1] => Fido [2] => Cat )     print_r(array_map("myFunction",$a));          //Array ( [0] => Horse [1] => Dog [2] => Cat )    //为什么回调函数在array_filter()里面没有起作用？    print_r(array_filter($a,"myFunction"));