The PHP documentation is not always so good with examples but in this case the imagefttext() docs should help.

Use opacity function link .

<?php

function filter_opacity(&$img, $opacity) //params: image resource id,  opacity in percentage (eg. 80)
{
    if (!isset($opacity)) {
        return false;
    }
    $opacity /= 100;

    //get image width and height
    $w = imagesx($img);
    $h = imagesy($img);

    //turn alpha blending off
    imagealphablending($img, false);

    //find the most opaque pixel in the image (the one with the smallest alpha value)
    $minalpha = 127;
    for ($x = 0; $x < $w; $x++) {
        for ($y = 0; $y < $h; $y++) {
            $alpha = (imagecolorat($img, $x, $y) >> 24) & 0xFF;
            if ($alpha < $minalpha) {
                $minalpha = $alpha;
            }
        }
    }

    //loop through image pixels and modify alpha for each
    for ($x = 0; $x < $w; $x++) {
        for ($y = 0; $y < $h; $y++) {
            //get current alpha value (represents the TANSPARENCY!)
            $colorxy = imagecolorat($img, $x, $y);
            $alpha = ($colorxy >> 24) & 0xFF;
            //calculate new alpha
            if ($minalpha !== 127) {
                $alpha = 127 + 127 * $opacity * ($alpha - 127) / (127 - $minalpha);
            } else {
                $alpha += 127 * $opacity;
            }
            //get the color index with new alpha
            $alphacolorxy = imagecolorallocatealpha($img, ($colorxy >> 16) & 0xFF, ($colorxy >> 8) & 0xFF, $colorxy & 0xFF, $alpha);
            //set pixel with the new color + opacity
            if (!imagesetpixel($img, $x, $y, $alphacolorxy)) {
                return false;
            }
        }
    }

    return true;
}

Example usage:

<?php
$image = imagecreatefrompng("img.png");
filter_opacity($image, 75);
header("content-type: image/png");
imagepng($image);
imagedestroy($image);