PHP proportional image resizing math 7/18/13

Recently I needed to enlarge and shrink images proportionally¹ in Wordpress², so I wrote a couple PHP functions to help with the math³. In both, you supply the original height and width, and then maximum or minimum dimensions. If the original height and width fit within your maximum or minimum, the functions return null, if not they return an array with new proportional dimensions.

<?php
function image_resize_up($orig_w, $orig_h, $MIN_W, $MIN_H){
    $ratio = $orig_w * 1.0 / $orig_h;
 
    $w_undersized = ($orig_w < $MIN_W);
    $h_undersized = ($orig_h < $MIN_H);
    
    if ($w_undersized OR $h_undersized)
    {
          $new_w = round( max($MIN_W, $ratio * $MIN_H) );
          $new_h = round( max($MIN_H, $MIN_W / $ratio) );
          return array('width' => $new_w, 'height' => $new_h);
    }
    return null;
}

function image_resize_down($orig_w, $orig_h, $MAX_W, $MAX_H){
    $ratio = $orig_w * 1.0 / $orig_h;
 
    $w_undersized = ($orig_w > $MAX_W);
    $h_undersized = ($orig_h > $MAX_H);
    
    if ($w_undersized OR $h_undersized)
    {
          $new_w = round( min($MAX_W, $ratio * $MAX_H) );
          $new_h = round( min($MAX_H, $MAX_W / $ratio) );
          return array('width' => $new_w, 'height' => $new_h);
    }
    return null;
}
?>

Resizing the images proportionally means that they will maintain their original [aspect ratio](https://en.wikipedia.org/wiki/Aspect_ratio_(image%29) and not look distorted. ↩
In Wordpress you can use registered image sizes to shrink images proportionally, but there is nothing for enlarging them. ↩
Extended and modified from this answer by jon_darkstar on Stack Overflow. ↩