// Define the folder to clean
// (keep trailing slashes)
$captchaFolder  = 'temp/';
 
// Filetypes to check (you can also use *.*)
$fileTypes      = '*.jpg';
 
// Here you can define after how many
// minutes the files should get deleted
$expire_time    = 20; 
 
// Find all files of the given file type
foreach (glob($captchaFolder . $fileTypes) as $Filename) {
 
    // Read file creation time
    $FileCreationTime = filectime($Filename);
 
    // Calculate file age in seconds
    $FileAge = time() - $FileCreationTime; 
 
    // Is the file older than the given time span?
    if ($FileAge > ($expire_time * 60)){
 
        // Now do something with the olders files...
 
        print "The file $Filename is older than $expire_time minutes\n";
 
        // For example deleting files:
        //unlink($Filename);
    }
 
}

PHP Variable to get the upload directory from config: get_cfg_var('upload_tmp_dir')

Delete temporary files

PHP