function scale_dimensions_within_limits($w,$h,$max_w,$max_h){
  // $w is the width of the current rectangle 
  // $h is the height of the current rectangle 
  // $max_w is the maximum width that an image can be sized 
  // $max_h is the maximum height that an image can be sized 

  // **** Here's where the magic is starts **** 
  // Switch the concept of horiz/vertical/square to long/short side 
  $short_side_len = ($w < $h ? $w : $h); 
  $long_side_len = ($w > $h ? $w : $h); 
  // Set a variable to the variable name of the output variable
  $ssvar = ($w > $h ? 'h':'w'); 
  $lsvar = ($w > $h ? 'w':'h'); 
  $maxLSvar = "max_".$lsvar; 
  $maxSSvar = "max_".$ssvar; 

  // Do the first pass on the long side
  $ratio = $$maxLSvar/$long_side_len; 
  $newSS = round($short_side_len * $ratio); 
  $newLS = round($long_side_len * $ratio); 

  // *** Note - the only coditional block!
  // If short side is still out of limit, limit the short side and adjust 
  if($newSS > $$maxSSvar){ 
    $ratio = $$maxSSvar/$newSS; 
    $newLS = round($ratio*$newLS); 
    $newSS = $$maxSSvar; 
  } 

  // **** Here's where the magic ends **** 
  // Re-couple the h/w (or w/h) with the long/shortside counterparts 
  // $$ means it's a variable variable (dynamic assignment) 
  $$ssvar = $newSS; 
  $$lsvar = $newLS;

  // Prep the return array 
  $dimensions['w'] = $w; // this is derived from either $ssvar or $lsvar 
  $dimensions['h'] = $h; return $dimensions; 
}

http://davesthings.posterous.com/im-feeling-brilliant

This is a much cleaner way to do this than the number of examples on the web.  Every example I saw seemed to account for orientation  - horizontal vs. vertical vs. square when looking at the input.  This ignores horizontal and vertical by mapping the "long side" and "short side" and then re-mapping the variables to $h / $w at the end.

Image Scaling within Limits (max height, max width)

php, image

PHP