A set of PHP functions to help get either white or black colour for foreground text over a given background

colour_contrast.php

<?php
/**
 * Determine if a black or white foreground color will have a better contrast
 * ratio for a given background color
 *
 * @param string|int $bg_r  A hex representation of the color
 *                          or the value of the red color 0-255
 * @param int $bg_g         The value of the green color 0-255
 * @param int $bg_b         The value of the blue color 0-255
 *
 * @return string|array  Either `#ffffff` or `#000000` if $bg_r was a string,
 *                       otherwise `[255, 255, 255]` or `[0, 0, 0]`
 */
function foreground_color($bg_r, $bg_g = null, $bg_b = null) {
    $is_string = is_string($bg_r);
    if ($is_string) {
        list($bg_r, $bg_g, $bg_b) = hex_to_rgb($bg_r);
    }
    $contrast = get_contrast($bg_r, $bg_g, $bg_b, 255, 255, 255);
    if ($contrast >= 4.5) {
        $result = [255, 255, 255];
    } else {
        $result = [0, 0, 0];
    }
    if ($is_string) {
        $result = array_map('dechex', $result);
        $result = array_map(function ($v) { return strlen($v) == 1 ? "{$v}{$v}" : $v; }, $result);
        return '#' . implode('', $result);
    }
    return $result;
}

/**
 * Compute contrast ratio between two colors according to WCAG 2.0 AA and AAA
 * levels
 *
 * A result between:
 *   - 0 and 3 means there is not enough contrast, thus fails WCAG 2.0 test
 *   - 3 and 4.5 means it passes AA level only for large text (above 18pt or
 *     bold above 14pt)
 *   - 4.5 and 7 means it passes AA level for any size text and AAA only for
 *     large text (above 18pt or bold above 14pt)
 *   - 7 and 22 means it passes AAA level for any size text
 *
 * @param int $r1  The value of the first colours' red 0-255
 * @param int $g1  The value of the first colours' green 0-255
 * @param int $b1  The value of the first colours' blue 0-255
 * @param int $r2  The value of the second colours' red 0-255
 * @param int $g2  The value of the second colours' green 0-255
 * @param int $b2  The value of the second colours' blue 0-255
 *
 * @return int  The contrast ratio between two colours
 */
function get_contrast($r1, $g1, $b1, $r2, $g2, $b2) {
    $lum1 = get_luminance($r1, $g1, $b1);
    $lum2 = get_luminance($r2, $g2, $b2);
    $ratio = ($lum1 + 0.05) / ($lum2 + 0.05);
    if ($lum2 > $lum1) {
        $ratio = 1 / $ratio;
    }
    return $ratio;
}

/**
 * Compute the relative luminance - the relative brightness of any point in a
 * colorspace, normalized to 0 for darkest black and 1 for lightest white
 *
 * @param int $r  The value of the red colour 0-255
 * @param int $g  The value of the green colour 0-255
 * @param int $b  The value of the blue colour 0-255
 *
 * @return int  The relative luminance of given colour
 */
function get_luminance($r, $g, $b) {
    $rgb = [$r, $g, $b];
    foreach ($rgb as $key => $val) {
        $val = $val / 255;
        $val = $val < 0.03928 ? $val / 12.92 : pow(($val + 0.055) / 1.055, 2.4);
        $rgb[$key] = $val;
    }
    return 0.2126 * $rgb[0] + 0.7152 * $rgb[1] + 0.0722 * $rgb[2];
}

/**
 * Convert a HEX color to a RGB array
 *
 * Both #RGB and #RRGGBB formats are supported, while # is optional.
 *
 * @param string $hex_color  A hex color
 *
 * @return array|FALSE  An [int, int, int] array or FALSE if format is incorrect.
 */
function hex_to_rgb($hex_color) {
    $hex_color = trim($hex_color, '#');
    $len = strlen($hex_color);
    if ($len == 3) {
        return array_map(function ($v) { return hexdec("{$v}{$v}"); }, str_split($hex_color));
    } else if ($len == 6) {
        return array_map('hexdec', str_split($hex_color, 2));
    }
    return FALSE;
}