function resize($cur_dir, $cur_file, $newwidth, $output_dir) {
    $dir_name = $cur_dir;
    $olddir = getcwd();
    $dir = opendir($dir_name);
    $filename = $cur_file;
    $format='';
    if(preg_match("/.jpg/i", "$filename"))
    {
        $format = 'image/jpeg';
    }
    if (preg_match("/.gif/i", "$filename"))
    {
        $format = 'image/gif';
    }
    if(preg_match("/.png/i", "$filename"))
    {
        $format = 'image/png';
    }
    if($format!='')
    {
        list($width, $height) = getimagesize($filename);
        $newheight=$height*$newwidth/$width;
        switch($format)
        {
            case 'image/jpeg':
            $source = imagecreatefromjpeg($filename);
            break;
            case 'image/gif';
            $source = imagecreatefromgif($filename);
            break;
            case 'image/png':
            $source = imagecreatefrompng($filename);
            break;
        }
        $thumb = imagecreatetruecolor($newwidth,$newheight);
        imagealphablending($thumb, false);
        $source = @imagecreatefromjpeg("$filename");
        imagecopyresized($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
        $filename="$output_dir/$filename";
        @imagejpeg($thumb, $filename);
    }
}
 
//call this function using 
resize("./input folder", "picture_file_name", "width", "./output folder");