Anyone wanting to create a reflecion of an image. Simple process that copies, line by line, from the bottom of the image, a given number of pixels. Each line gets gradually more transparent. Outputs PNG to screen.

This is pretty hot-coded - all four input variables (input image, reflection height, starting transparency, gap between image and reflection) are set manually here.

<?php
    $in = imagecreatefromjpeg('C:\test.jpg');
    $reflection_strength = 120;        //    starting transparency (0-127, 0 being opaque)
    $reflection_height = 40;        //     height of reflection in pixels
    $gap = 10;                        //    gap between image and reflection
    
    $orig_height = imagesy($in);                                //    store height of original image
    $orig_width = imagesx($in);                                    //    store height of original image
    $output_height = $orig_height + $reflection_height + $gap;    //    calculate height of output image
    
    // create new image to use for output. fill with transparency. ALPHA BLENDING MUST BE FALSE
    $out = imagecreatetruecolor($orig_width, $output_height);
    imagealphablending($out, false);
    $bg = imagecolortransparent($out, imagecolorallocatealpha($out, 255, 255, 255, 127));
    imagefill($out, 0, 0, $bg);
    imagefilledrectangle($out, 0, 0, imagesx($in), imagesy($in), $bg1);
    
    // copy original image onto new one, leaving space underneath for reflection and 'gap'
    imagecopyresampled ( $out , $in , 0, 0, 0, 0, imagesx($in), imagesy($in), imagesx($in), imagesy($in));

     // create new single-line image to act as buffer while applying transparency
    $reflection_section = imagecreatetruecolor(imagesx($in), 1);
    imagealphablending($reflection_section, false);
    $bg1 = imagecolortransparent($reflection_section, imagecolorallocatealpha($reflection_section, 255, 255, 255, 127));
    imagefill($reflection_section, 0, 0, $bg1);

    // 1. copy each line individually, starting at the 'bottom' of the image, working upwards. 
    // 2. set transparency to vary between reflection_strength and 127
    // 3. copy line back to mirrored position in original
    for ($y = 0; $y<$reflection_height;$y++)
    {    
        $t = ((127-$reflection_strength) + ($reflection_strength*($y/$reflection_height)));
        imagecopy($reflection_section, $out, 0, 0, 0, imagesy($in)  - $y, imagesx($in), 1);
        imagefilter($reflection_section, IMG_FILTER_COLORIZE, 0, 0, 0, $t);
        imagecopyresized($out, $reflection_section, $a, imagesy($in) + $y + $gap, 0, 0, imagesx($in) - (2*$a), 1, imagesx($in), 1);
    }
    
    // output image to view
    header('Content-type: image/png');
    imagesavealpha($out,true);
    imagepng($out);
?>