class_names.php

<?php
function array_reduce_allowed_str($array) {
    $reduced = '';
    array_walk($array, function($val, $key) use(&$reduced) {
        $reduced .= $val ? " $key" : '';
    });
    $cleaned = implode(' ', array_unique(array_map('trim', explode(' ', trim($reduced)))));
    return $cleaned;
}

function class_names($defaults, $classes = null, $failed = '') {
    if(!$result = array_reduce_allowed_str(is_array($defaults) ? $defaults : $classes)) {
        $result = !is_array($classes) ? $classes : $failed;
    }

    $defaults = !is_array($defaults) ? $defaults : '';

    return $defaults . ' ' . $result;
}

examples.md

class_names('hero flex align-center', [
    'full-bleed' => false,
    'full' => false
], 'full-bleed no-selection');
// outputs: hero flex align-center full-bleed no-selection

class_names('hero flex align-center', [
    'full-bleed' => false,
    'full' => true
], 'full-bleed no-selection')
// outputs: hero flex align-center full
    
class_names([
    'button-button button-shadowed button-shadowed-light' => false,
    'button-button button-shadowed button-shadowed-dark'  => false,
    'button-button button-plain-dark'                     => false,
    'button-button button-plain-light'                    => false,
    'button-with-icon'                                    => true
], 'button-button button-plain-dark')
// outputs: button-with-icon

class_names('hero flex align-center', [
    'full-bleed' => false,
    'full full full full' => true
], 'full-bleed no-selection')
// outputs: hero flex align-center full