php pdf文件转图片

为什么会文件读取不到，路径 pdf文件权限都已设置

$PDF = './22.pdf';

$IM =new imagick();
   $IM->setResolution(120,120);
   $IM->setCompressionQuality(100);
   $IM->readImage($PDF);
   foreach($IM as $Key => $Var){
       $Var->setImageFormat('png');
       $Filename = './'.md5($Key.time()).'.png';
       if($Var->writeImage($Filename)==true){
           $Return[]= $Filename;
       }
   }

报错内容：Fatal error: Uncaught exception 'ImagickException' with message 'Failed to read the file' in

---

try {
    // read img
}  catch (ImagickException $e) {
    echo  exception;
}

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$PDF使用绝对路径呢