<?php

//re-size an image and keep its exact proportions
//<img src="thumbnail.php?file=img.jpg&maxw=50&maxh=50" />

//Alternatively, we might want to have thumbnails that are all the same size, regardless of the original proportions. 
//<img src="thumbnail.php?file=img.jpg&width=50&height=50" />

 
// Get the File path for the image
 
$sImagePath = $_GET["file"];
 
// If you want exact dimensions, you
// will pass 'width' and 'height'
 
$iThumbnailWidth = (int)$_GET['width'];
$iThumbnailHeight = (int)$_GET['height'];
 
// If you want proportional thumbnails,
// you will pass 'maxw' and 'maxh'
 
$iMaxWidth = (int)$_GET["maxw"];
$iMaxHeight = (int)$_GET["maxh"];
 
// Based on the above we can tell which
// type of resizing our script must do
 
if ($iMaxWidth && $iMaxHeight) $sType = 'scale';
else if ($iThumbnailWidth && $iThumbnailHeight) $sType = 'exact';
 
// The 'scale' type will make the image
// smaller but keep the same dimensions
 
// The 'exact' type will make the thumbnail
// exactly the width and height you choose
 
// To start off, we will create a copy
// of our original image in $img
 
$img = NULL;
 
// At this point, we need to know the
// format of the image
 
// Based on that, we can create a new
// image using these functions:
// - imagecreatefromjpeg
// - imagecreatefrompng
// - imagecreatefromgif
 
$sExtension = strtolower(end(explode('.', $sImagePath)));
if ($sExtension == 'jpg' || $sExtension == 'jpeg') {
 
    $img = @imagecreatefromjpeg($sImagePath)
        or die("Cannot create new JPEG image");
 
} else if ($sExtension == 'png') {
 
    $img = @imagecreatefrompng($sImagePath)
        or die("Cannot create new PNG image");
 
} else if ($sExtension == 'gif') {
 
    $img = @imagecreatefromgif($sImagePath)
        or die("Cannot create new GIF image");
 
}
 
// If the image has been created, we may proceed
// to the next step
 
if ($img) {
 
    // We now need to decide how to resize the image
 
    // If we chose to scale down the image, we will
    // need to get the original image propertions
 
    $iOrigWidth = imagesx($img);
    $iOrigHeight = imagesy($img);
 
    if ($sType == 'scale') {
 
        // Get scale ratio
 
        $fScale = min($iMaxWidth/$iOrigWidth,
              $iMaxHeight/$iOrigHeight);
 
        // To explain how this works, say the original
        // dimensions were 200x100 and our max width
            // and height for a thumbnail is 50 pixels.
        // We would do $iMaxWidth/$iOrigWidth
        // (50/200) = 0.25
        // And $iMaxHeight/$iOrigHeight
        // (50/100) = 0.5
 
        // We then use the min() function
        // to find the lowest value.
 
        // In this case, 0.25 is the lowest so that
        // is our scale. The thumbnail must be
        // 1/4th (0.25) of the original image
 
        if ($fScale < 1) {
 
            // We must only run the code below
            // if the scale is lower than 1
            // If it isn't, this means that
            // the thumbnail we want is actually
            // bigger than the original image
 
            // Calculate the new height and width
            // based on the scale
 
            $iNewWidth = floor($fScale*$iOrigWidth);
            $iNewHeight = floor($fScale*$iOrigHeight);
            // Create a new temporary image using the
            // imagecreatetruecolor function
 
            $tmpimg = imagecreatetruecolor($iNewWidth,
                               $iNewHeight);
 
            // The function below copies the original
            // image and re-samples it into the new one
            // using the new width and height
 
            imagecopyresampled($tmpimg, $img, 0, 0, 0, 0,
            $iNewWidth, $iNewHeight, $iOrigWidth, $iOrigHeight);
 
            // Finally, we simply destroy the $img file
            // which contained our original image
            // so we can replace with the new thumbnail
 
            imagedestroy($img);
            $img = $tmpimg;
        }     
 
    } else if ($sType == "exact") {
 
        // Get scale ratio
 
        $fScale = max($iThumbnailWidth/$iOrigWidth,
              $iThumbnailHeight/$iOrigHeight);
 
        // This works similarly to other one but
        // rather than the lowest value, we need
        // the highest. For example, if the
        // dimensions were 200x100 and our thumbnail
        // had to be 50x50, we would calculate:
        // $iThumbnailWidth/$iOrigWidth
        // (50/200) = 0.25
        // And $iThumbnailHeight/$iOrigHeight
        // (50/100) = 0.5
 
        // We then use the max() function
        // to find the highest value.
 
        // In this case, 0.5 is the highest so that
        // is our scale. This is the first step of
        // the image manipulation. Once we scale
        // the image down to 0.5, it will have the
        // dimensions of 100x50. At this point,
        // we will need to crop the image, leaving
        // the height identical but halving
        // the width to 50
 
        if ($fScale < 1) {
 
            // Calculate the new height and width
            // based on the scale
 
            $iNewWidth = floor($fScale*$iOrigWidth);
            $iNewHeight = floor($fScale*$iOrigHeight);
            // Create a new temporary image using the
            // imagecreatetruecolor function
 
            $tmpimg = imagecreatetruecolor($iNewWidth,
                            $iNewHeight);
            $tmp2img = imagecreatetruecolor($iThumbnailWidth,
                            $iThumbnailHeight);
 
            // The function below copies the original
            // image and re-samples it into the new one
            // using the new width and height
 
            imagecopyresampled($tmpimg, $img, 0, 0, 0, 0,
            $iNewWidth, $iNewHeight, $iOrigWidth, $iOrigHeight);
 
            // Our $tmpimg will now have the scaled down
            // image. The next step is cropping the picture to
            // make sure it's exactly the size of the thumbnail
 
            // The following logic choose how the image
            // will be cropped. Using the previous example, it
            // needs to take a 50x50 block from the original
            // image and copy it over to the new thumbnail
 
            // Since we want to copy the exact center of the
            // scaled down image, we need to find out the x
            // axis and y axis. To do so, say the scaled down
            // image now has a width of 100px but we want it
            // to be only 50px
 
            // Somehow, we need to select between the 25th and
            // 75th pixel to copy the middle.
 
            // To find this value we do:
            // ($iNewWidth/2)-($iThumbnailWidth/2)
 
            // ( 100px / 2 ) - (50px / 2)
            // ( 50px ) - ( 25px )
            // = 25px
 
            if ($iNewWidth == $iThumbnailWidth) {
 
                $yAxis = ($iNewHeight/2)-
                    ($iThumbnailHeight/2);
                $xAxis = 0;
 
            } else if ($iNewHeight == $iThumbnailHeight)  {
 
                $yAxis = 0;
                $xAxis = ($iNewWidth/2)-
                    ($iThumbnailWidth/2);
 
            } 
 
            // We now have to resample the new image using the
            // new dimensions are axis values.
 
            imagecopyresampled($tmp2img, $tmpimg, 0, 0,
                       $xAxis, $yAxis,
                       $iThumbnailWidth,
                       $iThumbnailHeight,
                       $iThumbnailWidth,
                       $iThumbnailHeight);
 
            imagedestroy($img);
            imagedestroy($tmpimg);
            $img = $tmp2img;
        }    
 
    }
 
    // Display the image using the header function to specify
    // the type of output our page is giving
 
    header("Content-type: image/jpeg");
    imagejpeg($img);
 
}
 
?>

http://www.webgeekly.com/tutorials/php/how-to-create-an-image-thumbnail-on-the-fly-using-php/

The script that creates the thumbnail wlll also need to output it in an image format. This means we will be using the ordinary <img> tag which links to a PHP file rather than an image.

Image Thumbnail On-The-Fly Using PHP

PHP