PHP: determine legibility on different colors

readme.md

Using Mark Ransom's formula to determine the contrast.

Using arosolino's code to convert the hexadecimal value to Red, Green, Blue values.

functions.php

<?php

$hex = "#A0DAA9"; // accepts abc, aabbcc, #abc and #aabbcc.

// Split HEX value into Red Green and Blue values
list( $r, $g, $b ) = array_map(
  function ( $c ) {
    return hexdec( str_pad( $c, 2, $c ) );
  },
  str_split(
    ltrim( $hex, '#' ),
    strlen( $hex ) > 4 ? 2 : 1 )
);

// Multiply with industry standards for contrast values
$red   = ( $r * 0.299 );
$green = ( $g * 0.587 );
$blue  = ( $b * 0.114 );

// Combine RGB values
$rgb   = ( $r * 0.299 ) + ( $g * 0.587 ) + ( $b * 0.114 );

// advised contrast limit for displays
$limit = 186;

if( $rgb >= $limit ) {
  // $hex is bright, text should be dark
  $text = "#000";
} else {
  // $hex is dark, text should be bright
  $text = "#fff";
}

// Oneliner version
$text = ( ( $r * 0.299 ) + ( $g * 0.587 ) + ( $b * 0.114 ) >= 186 ) ? "#000" : "#fff" ;

// $text (color) is now a hexadecimal value that is readable on $hex (color).