Here is a function that takes an image ($im) and returns it with the contrast maximised...

<?php
function contrast($im) {
    $brightness=0;
    $maxb=0;
    $minb=255;
    $imagesize=getimagesize($im);
    $w=$imagesize[0];
    $h=$imagesize[1];
    for ($x=0; $x<$w; $x++) {
        for ($y=0; $y<$h; $y++) {
            $rgb=imagecolorat($im, $x, $y);
            $rgb=imagecolorsforindex($im, $rgb);
            $grey=0.2125*$rgb['red']+
                0.7154*$rgb['green']+
                0.0721*$rgb['blue']; 
            $brightness+=$grey;
            if ($grey>$maxb) $maxb=$grey;
            if ($grey<$minb) $minb=$grey;
        }
    }
    $brightness=$brightness/($w*$h);
    $minb=$brightness/($brightness-$minb);
    $maxb=(255-$brightness)/($maxb-$brightness);
    $contrast=min($minb, $maxb);
    for ($x=0; $x<$w; $x++) {
        for ($y=0; $y<$h; $y++) {
            $rgb=imagecolorat($im, $x, $y);
            $rgb=imagecolorsforindex($im, $rgb);
            imagesetpixel($im, $x, $y,
                65536*floor(min($rgb['red']*$contrast, 255))+
                256*floor(min($rgb['green']*$contrast, 255))+
                floor(min($rgb['blue']*$contrast, 255)));
        }
    }
    return ($im);
}
?>

An example of usage might be:
<?php
$imagefile="/path/filename";
$image=imagecreatefromjpeg($imagefile);
$image=contrast($image);
imagejpeg($image, $imagefile);
?>