RosettaCodeData/Task/Bitmap-Histogram/PHP/bitmap-histogram.php

define('src_name', 'input.jpg');  // source image
define('dest_name', 'output.jpg');  // destination image

$img = imagecreatefromjpeg(src_name); // read image

if(empty($img)){
  echo 'Image could not be loaded!';
  exit;
}

$black = imagecolorallocate($img, 0, 0, 0);
$white = imagecolorallocate($img, 255, 255, 255);
$width = imagesx($img);
$height = imagesy($img);

$array_lum = array();   // for storage of luminosity of each pixel
$sum_lum = 0;   // total sum of luminosity
$average_lum = 0; // average luminosity of whole image

for($x = 0; $x < $width; $x++){
  for($y = 0; $y < $height; $y++){
    // read pixel value
    $color = imagecolorat($img, $x, $y);
    $r = ($color >> 16) & 0xFF;
    $g = ($color >> 8) & 0xFF;
    $b = $color & 0xFF;
    // save pixel luminosity in temporary array
    $array_lum[$x][$y] = ($r + $g + $b);
    // add pixel luminosity to sum
    $sum_lum += $array_lum[$x][$y];
  }
}

// calculate average luminosity
$average_lum = $sum_lum / ($width * $height);

for($x = 0; $x < $width; $x++){
  for($y = 0; $y < $height; $y++){
    // pixel is brighter than average -> set white
    // else -> set black
    if($array_lum[$x][$y] > $average_lum){
      imagesetpixel($img, $x, $y, $white);
    }
    else{
      imagesetpixel($img, $x, $y, $black);
    }
  }
}
// save black and white image to dest_name
imagejpeg($img, dest_name);

if(!file_exists(dest_name)){
  echo 'Image not saved! Check permission!';
}