php 에서 shell_exec 실행후 값을 가져오려는데 잘 안되네요

function callCmdLine($data, $imgFile, $dirname, $filename_prefix) {
    global $sdeditJar, $QSDEImagetype;
 
    // create temporary uml text file
    $umlFile = $dirname."/".$filename_prefix.".sml";
    $fp = fopen($umlFile,"w+");
    $w = fputs($fp,$data);
    fclose($fp);
 
    // Lauch PlantUML
    if ($QSDEImagetype == 'png') {
        $typestr = ' -t png';
    } else {
        die (-1);
    }
 
    $command = "java -jar ". $sdeditJar. "{$typestr}". ' '.  '-o'. "\"{$imgFile}\"". ' '. "\"{$umlFile}\"";
 
    /** Debugging
     * $status_code = system($command);
     */
 
    $status_code = exec($command);
    /* let us use system command to display error */
 
    // Delete temporary uml text file
    unlink($umlFile);
 
    // Only return existing path names.
    if (is_file($imgFile)) {
        return $imgFile;
    } else {
        return false;
   }
}