<?php
    set_time_limit(0);

    $magick_commands = array ('gm convert', 'convert');

    $image_rendition_geometries = array(
        '138',
        '240',
        '400',
        '507',
        '800',
        '1034',
        '1200',
        '1600',
        '2000'
    );

    // Make sure you have input.png
    $input_file = 'input.png';

    // Since GraphicsMagick and ImageMagick accepts the same parameters, just iterate
    // over the commands and concatenate with the same paramters.
    foreach ($magick_commands as $magick_command)
    {
        $start = getTime();

        // Iterate through the process several times to get a good average
        for ($i = 1; $i <= 10; $i++)
        {
            // Iterate over the defined geometries. Having multiple geometries
            // allows assement of average performance at different geometries
            foreach ($image_rendition_geometries as $image_rendition_geometry)
            {
                // Create output filename like this: 'gm convert-138.png', 'convert-2000.png', etc.
                $output_file = "'$magick_command-$image_rendition_geometry" . ".png'";

                // Execute magick command
                $command = "$magick_command -units PixelsPerInch -density 96x96 -quality 100 -resize " . $image_rendition_geometry . "x $input_file $output_file";
                exec($command);
            }
        }

        $end =  getTime();
        $time_difference = round($end - $start, 4);

        $iteration = $i - 1;
        echo ("'$magick_command' took $time_difference seconds to execute $iteration iteration(s). <br/>");
    }

    // Get filesize of the generated images
    reset($magick_commands);
    $file_sizes = array();
    foreach ($magick_commands as $magick_command)
    {
        reset($image_rendition_geometries);
        foreach ($image_rendition_geometries as $image_rendition_geometry)
        {
            $output_file = "$magick_command-$image_rendition_geometry" . ".png";
            $file_sizes[$magick_command][$image_rendition_geometry] = filesize($output_file);
        }
    }

    // Display the average file sizes
    reset($magick_commands);
    foreach ($magick_commands as $magick_command)
    {
        print_r("Average filesize of $magick_command: " .  number_format(array_sum($file_sizes[$magick_command]) / count($image_rendition_geometries), 0, '', ',') . ' bytes. <br/>');
    }

    function getTime() {
        $timer = explode( ' ', microtime() );
        $timer = $timer[1] + $timer[0];
        return $timer;
    }