exec命令无法在imagemagick php中工作

Question

我正在尝试使用imagemagick并排加入两张图片。

        $in_file1 = escapeshellarg(trim($dir."_thumb/demo-".($r).".jpg"));
        $in_file2 = escapeshellarg(trim($dir."_thumb/demo-".($r+1).".jpg"));
        $out_put_file = escapeshellarg(trim($dir."_thumb/demo-horiz-".($i).".jpg"));
        //echo "convert  $in_file1 $in_file2 +append $out_put_file<hr/>";

        //$cmd = "convert ".$in_file1." ". $in_file2. " +append ".$out_put_file;
        //echo $cmd."<br/><hr/>";
        //exec("convert julycacae_thumb/demo-13.jpg julycacae_thumb/demo-14.jpg +append julycacae_thumb/demo-horiz-6.jpg"); 


            $fmt="convert %s %s +append %s";

            $cmd=sprintf($fmt, $in_file1, $in_file2, $out_put_file);
            echo $cmd."<br/><hr/>";
            exec($cmd, $dimm_loc, $ipmiretval);
            syslog(LOG_DEBUG, "Running: $cmd");

            if ($ipmiretval > 0) {
              syslog(LOG_ERR, "exec FAILED: $cmd");
            } else {
              syslog(LOG_DEBUG, "exec: $cmd");
            }

        exec("convert julycdaad_thumb/demo-15.jpg julycdaad_thumb/demo-16.jpg +append julycdaad_thumb/demo-horiz-7.jpg"); 
        $r = $r+2;'

当我在exec中编写完整的图像路径时，它可以正常工作，但是在传递变量时它会有效...