Get all icon classes of font awesome

icons.php

<?php

$json = json_decode(file_get_contents('icons.json'));
$icons = [];

foreach ($json as $class => $icon) {
    $name = 'fa-' . $class;

    foreach ($icon->styles as $type) {
        switch($type) {
            case 'solid':
                $c = 'fas ' . $name;
                $icons[$c] = $icon->unicode;
                break;
            case 'regular':
                $c = 'far ' . $name;
                $icons[$c] = $icon->unicode;
                break;
            case 'brands':
                $c = 'fab ' . $name;
                $icons[$c] = $icon->unicode;
                break;
        }
    }
}

echo "<pre>";

print_r($icons);

echo "</pre><br>";

echo count($icons);

?>

icons.txt

The icons.json you can get under https://fontawesome.com/how-to-use/on-the-web/setup/hosting-font-awesome-yourself.
Open the ZIP archive and navigate to metadata > icons.json.