PHP upload file type determination function of to avoid upload vulnerability

function ($file_name,$pass_type=array('jpg','jpeg','gif','bmp','png')){ 
$yx_file = $pass_type; 
$kzm = substr(strrchr($file_name,"."),1); 
$is_img = in_array(strtolower($kzm),$yx_file); 
if($is_img){ 
return true; 
}else{ 
return false; 
} 
}