If you would like to resize your image with good quality. Please use the below function to resize you image.

$img = 'var/www/html/myproject/images/mypicture.jpg';
resize_image($img,'',320,320);

 function resize_image(
                              $filepath,
                              $string             = null,
                              $width              = 200,
                              $height             = 200,
                              $proportional       = false,
                              $output             = 'file',
                              $delete_original    = true,
                              $use_linux_commands = false,
                              $quality = 100
   ) {

    if ( $height <= 0 && $width <= 0 ) return false;
    if ( $file === null && $string === null ) return false;

    # Setting defaults and meta
    $info                         = $file !== null ? getimagesize($file) : getimagesizefromstring($string);
    $image                        = '';
    $final_width                  = 0;
    $final_height                 = 0;
    list($width_old, $height_old) = $info;
$cropHeight = $cropWidth = 0;

    # Proportional calculation
    if ($proportional) {
      if      ($width  == 0)  $factor = $height/$height_old;
      elseif  ($height == 0)  $factor = $width/$width_old;
      else                    $factor = min( $width / $width_old, $height / $height_old );

      $final_width  = round( $width_old * $factor );
      $final_height = round( $height_old * $factor );
    }
    else {
      $final_width = ( $width <= 0 ) ? $width_old : $width;
      $final_height = ( $height <= 0 ) ? $height_old : $height;
  $widthX = $width_old / $width;
  $heightX = $height_old / $height;

  $x = min($widthX, $heightX);
  $cropWidth = ($width_old - $width * $x) / 2;
  $cropHeight = ($height_old - $height * $x) / 2;
    }

    # setting as per image extension 
    switch ( $info[2] ) {
      case IMAGETYPE_JPEG:  $file !== null ? $image = imagecreatefromjpeg($file) : $image = imagecreatefromstring($string);  break;
      case IMAGETYPE_GIF:   $file !== null ? $image = imagecreatefromgif($file)  : $image = imagecreatefromstring($string);  break;
      case IMAGETYPE_PNG:   $file !== null ? $image = imagecreatefrompng($file)  : $image = imagecreatefromstring($string);  break;
      default: return false;
    }


    # Now we are going to resize 
    $image_resized = imagecreatetruecolor( $final_width, $final_height );
    if ( ($info[2] == IMAGETYPE_GIF) || ($info[2] == IMAGETYPE_PNG) ) {
      $transparency = imagecolortransparent($image);
      $palletsize = imagecolorstotal($image);

      if ($transparency >= 0 && $transparency < $palletsize) {
        $transparent_color  = imagecolorsforindex($image, $transparency);
        $transparency       = imagecolorallocate($image_resized, $transparent_color['red'], $transparent_color['green'], $transparent_color['blue']);
        imagefill($image_resized, 0, 0, $transparency);
        imagecolortransparent($image_resized, $transparency);
      }
      elseif ($info[2] == IMAGETYPE_PNG) {
        imagealphablending($image_resized, false);
        $color = imagecolorallocatealpha($image_resized, 0, 0, 0, 127);
        imagefill($image_resized, 0, 0, $color);
        imagesavealpha($image_resized, true);
      }
    }
    imagecopyresampled($image_resized, $image, 0, 0, $cropWidth, $cropHeight, $final_width, $final_height, $width_old - 2 * $cropWidth, $height_old - 2 * $cropHeight);
    # Taking care of original, if needed
    if ( $delete_original ) {
      if ( $use_linux_commands ) exec('rm '.$file);
      else @unlink($file);
    }

    # Preparing a method of providing result
    switch ( strtolower($output) ) {
      case 'browser':
        $mime = image_type_to_mime_type($info[2]);
        header("Content-type: $mime");
        $output = NULL;
      break;
      case 'file':
        $output = $file;
      break;
      case 'return':
        return $image_resized;
      break;
      default:
      break;
    }

    # Setting image according to dimension and quality
    switch ( $info[2] ) {
      case IMAGETYPE_GIF:   imagegif($image_resized, $output);    break;
      case IMAGETYPE_JPEG:  imagejpeg($image_resized, $output, $quality);   break;
      case IMAGETYPE_PNG:
        $quality = 9 - (int)((0.9*$quality)/10.0);
        imagepng($image_resized, $output, $quality);
        break;
      default: return false;
    }

    return true;
  }