PHP function to auto-pick black/white text color

pick-on-color.php

	<?php
  
	/**
	 * Calculate optimal foreground (#ffffff or #000000) for a given hex background.
	 *
	 * Uses WCAG relative luminance; threshold where contrast with white equals
	 * contrast with black is approximately 0.179.
	 *
	 * @since 1.0.0
	 *
	 * @param string $hex Background color in hex format ('#abc', 'abc', '#aabbcc', 'aabbcc').
	 * @return string '#ffffff' or '#000000'.
	 */
	public function pick_on_color( $hex ) {
		$hex = ltrim( trim( (string) $hex ), '#' );

		if ( strlen( $hex ) === 3 ) {
			$hex = $hex[0] . $hex[0] . $hex[1] . $hex[1] . $hex[2] . $hex[2];
		}

		if ( strlen( $hex ) !== 6 || ! ctype_xdigit( $hex ) ) {
			return '#000000';
		}

		$r = hexdec( substr( $hex, 0, 2 ) ) / 255;
		$g = hexdec( substr( $hex, 2, 2 ) ) / 255;
		$b = hexdec( substr( $hex, 4, 2 ) ) / 255;

		$r = ( $r <= 0.03928 ) ? ( $r / 12.92 ) : pow( ( $r + 0.055 ) / 1.055, 2.4 );
		$g = ( $g <= 0.03928 ) ? ( $g / 12.92 ) : pow( ( $g + 0.055 ) / 1.055, 2.4 );
		$b = ( $b <= 0.03928 ) ? ( $b / 12.92 ) : pow( ( $b + 0.055 ) / 1.055, 2.4 );

		$l = ( 0.2126 * $r ) + ( 0.7152 * $g ) + ( 0.0722 * $b );

		return ( $l <= 0.179 ) ? '#ffffff' : '#000000';
	}