Snippet - function to Increase/Decrease brightness of a HEX color by a percentage

functions.php

<?php

/**
 * Increases or decreases the brightness of a color by a percentage of the current brightness.
 *
 * Credit: https://stackoverflow.com/a/54393956/4256497
 *
 * @param   string  $hex_code        Supported formats: `#FFF`, `#FFFFFF`, `FFF`, `FFFFFF`
 * @param   float   $adjust_pct  A number between -1 and 1. E.g. 0.3 = 30% lighter; -0.4 = 40% darker.
 *
 * @return  string
 */
function adjust_brightness( $hex_code, $adjust_pct ) {

	$hex_code = ltrim( $hex_code, '#' );

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

	$hex_code = array_map( 'hexdec', str_split( $hex_code, 2 ) );

	foreach ( $hex_code as & $color ) {
		$adjustable_limit = $adjust_pct < 0 ? $color : 255 - $color;
		$adjust_amount = ceil( $adjustable_limit * $adjust_pct );

		$color = str_pad( dechex( $color + $adjust_amount ), 2, '0', STR_PAD_LEFT );
	}

	return '#' . implode( $hex_code );
}