Here is an example of how to resize an image to a certain % of it's original size, and keep it proportional. Pay attention to the comments in the code, as they explain what we are doing in each step.
<?php
// The file you are resizing
$file = 'your.jpg';
//This will set our output to 45% of the original size
$size = 0.45;
// This sets it to a .jpg, but you can change this to png or gif
header('Content-type: image/jpeg');
// Setting the resize parameters
list($width, $height) = getimagesize($file);
$modwidth = $width * $size;
$modheight = $height * $size;
// Creating the Canvas
$tn= imagecreatetruecolor($modwidth, $modheight);
$source = imagecreatefromjpeg($file);
// Resizing our image to fit the canvas
imagecopyresized($tn, $source, 0, 0, 0, 0, $modwidth, $modheight, $width, $height);
// Outputs a jpg image, you could change this to gif or png if needed
imagejpeg($tn);
?>
