Use PHP to create thumbnails. (Cropped to square)

I have a php script im currently using that creates thumbnails based on a max width and height. However, I'd like it to always create square images and crop the images when needed.

Here is what I'm using now:

    function makeThumb( $filename, $type ) {
  global $max_width, $max_height;
  if ( $type == 'jpg' ) {
   $src = imagecreatefromjpeg("blocks/img/gallery/" . $filename);
  } else if ( $type == 'png' ) {
   $src = imagecreatefrompng("blocks/img/gallery/" . $filename);
  } else if ( $type == 'gif' ) {
   $src = imagecreatefromgif("blocks/img/gallery/" . $filename);
  }
  if ( ($oldW = imagesx($src)) < ($oldH = imagesy($src)) ) {
   $newW = $oldW * ($max_width / $oldH);
   $newH = $max_height;
  } else {
   $newW = $max_width;
   $newH = $oldH * ($max_height / $oldW);
  }
  $new = imagecreatetruecolor($newW, $newH);
  imagecopyresampled($new, $src, 0, 0, 0, 0, $newW, $newH, $oldW, $oldH);
  if ( $type == 'jpg' ) {
   imagejpeg($new, 'blocks/img/gallery/thumbs/'.$filename);
  } else if ( $type == 'png' ) {
   imagepng($new, 'blocks/img/gallery/thumbs/'.$filename);
  } else if ( $type == 'gif' ) {
   imagegif($new, 'blocks/img/gallery/thumbs/'.$filename);
  }
  imagedestroy($new);
  imagedestroy($src);
 }

How would I alter this to accomplish what I want (Square thumbs)?

Thanks in advance.

function makeThumb( $filename , $thumbSize=100 ){
  global $max_width, $max_height;

  // Set Filenames
  $srcFile = 'blocks/img/gallery/'.$filename;
  $thumbFile = 'blocks/img/gallery/thumbs/'.$filename;

  // Determine the File Type
  $type = substr( $filename , strrpos( $filename , '.' )+1 );

  // Create the Source Image
  switch( $type ){
    case 'jpg' :
    case 'jpeg' :
      $src = imagecreatefromjpeg( $srcFile ); break;
    case 'png' :
      $src = imagecreatefrompng( $srcFile ); break;
    case 'gif' :
      $src = imagecreatefromgif( $srcFile ); break;
  }

  // Determine the Image Dimensions
  $oldW = imagesx( $src );
  $oldH = imagesy( $src );

  // Calculate the New Image Dimensions
  $limiting_dim = 0;

  if( $oldH > $oldW ){
    // Portrait
    $limiting_dim = $oldW;
  }else{
    // Landscape
    $limiting_dim = $oldH;
  }

  // Create the New Image
  $new = imagecreatetruecolor( $thumbSize , $thumbSize );

  // Transcribe the Source Image into the New (Square) Image
  imagecopyresampled( $new , $src , 0 , 0 , ($oldW-$limiting_dim )/2 , ( $oldH-$limiting_dim )/2 , $thumbSize , $thumbSize , $limiting_dim , $limiting_dim );

  // Output the image to the file
  switch( $type ){
    case 'jpg' :
    case 'jpeg' :
      $src = imagejpeg( $new , $thumbFile ); break;
    case 'png' :
      $src = imagepng( $new , $thumbFile ); break;
    case 'gif' :
      $src = imagegif( $new , $thumbFile ); break;
  }

  // Free up the memory
  imagedestroy( $new );
  imagedestroy( $src );
}

Hat Tip to @Sam Wan for the adjusted solution. Sorry it took me so long to incorporate it into this answer.