为啥回调函数在array_filter()里面没有起作用

<br />

    <?php<br />

        function myFunction($v) <br />

        {<br />

        if ($v==="Dog")<br />

            {<br />

                return "Fido";<br />

            }<br />

                return $v;<br />

        }<br />

        $a=array("Horse","Dog","Cat");<br />

        //Array ( [0] => Horse [1] => Fido [2] => Cat ) <br />

        print_r(array_map("myFunction",$a)); <br />

        <br />

        //Array ( [0] => Horse [1] => Dog [2] => Cat )<br />

        //为什么回调函数在array_filter()里面没有起作用？<br />

        print_r(array_filter($a,"myFunction"));<br />

     ?><br />