<?php
$target = 29350.4;
$tmpfile = '/tmp/brightness.jpg';

foreach (glob("*JPG") as $file) {
echo "Processing $file \n";

$out = shell_exec("identify  -format \"%[mean]\" $file");
$brightness = floatval($out);
echo "Initial brightness: $brightness\n";

//set up initial range
if($brightness > $target) {
//we need to darken the photo
$min = -4;
$max = 0;
} else {
//we need to make it brighter
$min = 0;
$max = 4;
}

$i = 0;
do {
$current = ($min + $max) / 2;
$brightness = adjust($current, $file);
$diff = $target - $brightness;
echo "Current value: $current\n";
echo "Current brightness: $brightness\n";
echo "Current diff: $diff\n";
if($diff > 0) {
//we need to make it lighter
$min = $current;
} else {
//we need to make it darker
$max = $current;
}
$i++;
} while(abs($diff) > 200);

//overwrite original image with adjusted tmpfile
unlink($file);
rename($tmpfile,$file);
}

function adjust($value, $file) {
global $tmpfile;
@unlink($tmpfile);
if($value > 0) {
shell_exec("convert $file -sigmoidal-contrast '$value,0%' $tmpfile");
} else {
shell_exec("convert $file +sigmoidal-contrast '$value,0%' $tmpfile");
}
$out = shell_exec("identify  -format \"%[mean]\" $tmpfile");
return floatval($out);
}