Gets a contrasting color based on YIQ equation - Source: https://24ways.org/2010/calculating-color-contrast/

getContrastYIQ.js

function getContrastYIQ(hexcolor){
	var r = parseInt(hexcolor.substr(0,2),16);
	var g = parseInt(hexcolor.substr(2,2),16);
	var b = parseInt(hexcolor.substr(4,2),16);
	var yiq = ((r*299)+(g*587)+(b*114))/1000;
	return (yiq >= 128) ? 'black' : 'white';
}

getContrastYIQ.php

<?php

  function getContrastYIQ($hexcolor){
    $r = hexdec(substr($hexcolor,0,2));
    $g = hexdec(substr($hexcolor,2,2));
    $b = hexdec(substr($hexcolor,4,2));
    $yiq = (($r*299)+($g*587)+($b*114))/1000;
    return ($yiq >= 128) ? 'black' : 'white';
  }