为啥回调函数在array_filter()里面没有起作用

为什么回调函数在array_filter()里面没有起作用？

  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"));