Question: I want users on my website to be able to pick a hex colour, and I just want to display white text for dark colours and black text for light colours. Can you work out the brightness from a hex code (preferably PHP)?

Answer: 
I made one similar - but based on weightings of each colour [(based on the C# version of this thread)](https://stackoverflow.com/a/2241471/174034)

```
function readableColour($bg){
    $r = hexdec(substr($bg,0,2));
    $g = hexdec(substr($bg,2,2));
    $b = hexdec(substr($bg,4,2));

    $contrast = sqrt(
        $r * $r * .241 +
        $g * $g * .691 +
        $b * $b * .068
    );

    if($contrast > 130){
        return '000000';
    }else{
        return 'FFFFFF';
    }
}

echo readableColour('000000'); // Output - FFFFFF

```

**EDIT:** 
Small optimisation:
Sqrt is known as an expensive math operation, which is probably neglectable in most scenarios, but anyway, it could be avoided by doing something like this.

```
function readableColour($bg){
    $r = hexdec(substr($bg,0,2));
    $g = hexdec(substr($bg,2,2));
    $b = hexdec(substr($bg,4,2));

    $squared_contrast = (
        $r * $r * .299 +
        $g * $g * .587 +
        $b * $b * .114
    );

    if($squared_contrast > pow(130, 2)){
        return '000000';
    }else{
        return 'FFFFFF';
    }
}

echo readableColour('000000'); // Output - FFFFFF

```

It simply doesn't apply the sqrt, instead it powers the desired cut off contrast by two, which is a much cheaper calculation
I tried a different approach to this, I used HSL (hue, saturation & lightness) lightness percentage to check if the color is dark or light. (like @chrisf said in his answer)

**function:**

```
function colorislight($hex) {
   $hex       = str_replace('#', '', $hex);
   $r         = (hexdec(substr($hex, 0, 2)) / 255);
   $g         = (hexdec(substr($hex, 2, 2)) / 255);
   $b         = (hexdec(substr($hex, 4, 2)) / 255);
   $lightness = round((((max($r, $g, $b) + min($r, $g, $b)) / 2) * 100));
   return ($lightness >= 50 ? true : false);
}

```

On the return line it checks if the lightness percentage is higher than 50% and returns true otherwise false is returned. You can easily change it to return true if the color has 30% lightness and so on. The `$lightness` variable can return from 0 to 100 0 being the darkest and 100 being the lightest.

**how to use the function:**

```
$color = '#111111';
if ( colorislight($color) ) {
   echo 'this color is light';
}
else {
   echo 'this color is dark';
}

```