为啥回调函数在array_filter()里面没有起作用

function myFunction($v) <br />
    {<br />
    if ($v==="Dog")<br />
        {<br />
            return true;<br />
        }<br />
            return false;<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"));