PHP CLI script in procedural style

cli.php

<?php

function writeln($message = '', $stream = STDOUT)
{
    fwrite($stream, $message . PHP_EOL);
}

$arg_options = array(
    'help' => array(
        'required' => false,
        'description' => 'Displays the help message',
    ),
    'zipcode' => array(
        'required' => true,
        'description' => 'Zip code',
        'validator' => function ($zip_code)
        {
            return (1 == preg_match('/\d{5}-?\d{3}/', $zip_code));
        },
        'filter' => function ($zip_code)
        {
            return preg_replace('/[^\d]/', '', $zip_code);
        }
    ),
    'filename' => array(
        'required' => false,
        'description' => 'File to read',
        'default' => __DIR__ . '/address.xml',
        'validator' => 'is_file',
    )
);

$help_msg   = 'Usage: php '. array_shift($_SERVER['argv']) . ' OPTIONS' . PHP_EOL . PHP_EOL;
foreach ($arg_options as $key => $value) {
    $help_msg .= '    --' . str_pad($key, 10) . $value['description'] . PHP_EOL;
}

$arg_values = array();
foreach ($_SERVER['argv'] as $key => $value) {

    if (0 != $key % 2 ) {
        continue;
    }

    $arg_key = substr($value, 2);
    if (!isset($arg_options[$arg_key])) {
        writeln('Unrecognized option ' . $value, STDERR);
        exit (2);
    }
    $arg_option = $arg_options[$arg_key];

    if (isset($_SERVER['argv'][$key + 1])) {
        $arg_value = $_SERVER['argv'][$key + 1];
    } elseif (isset($arg_option['default'])) {
        $arg_value = $arg_option['default'];
    } else {
        $arg_value = true;
    }

    $arg_values[$arg_key]  = $arg_value;
}

if (empty($arg_values)) {
    writeln($help_msg, STDERR);
    exit (1);
}

if (isset($arg_values['help'])) {
    writeln($help_msg);
    exit;
}

foreach ($arg_options as $arg_key => $arg_option) {

    if (isset($arg_option['default'])
        && !isset($arg_values[$arg_key])) {
        $arg_values[$arg_key] = $arg_option['default'];
    }

    $arg_value = isset($arg_values[$arg_key]) ? $arg_values[$arg_key] : null;

    if (null === $arg_value && true === $arg_option['required']) {
        writeln('Value for "--' . $arg_key . '" is required', STDERR);
        exit (3);
    }

    if (null !== $arg_value && isset($arg_option['validator'])) {

        if (!call_user_func($arg_option['validator'], $arg_value)) {
            writeln('Value for "--' . $arg_key  . '" is not valid', STDERR);
            exit (4);
        }

    }

    if (isset($arg_option['filter'])) {
        $arg_value = call_user_func($arg_option['filter'], $arg_value);
    }

    $arg_values[$arg_key]  = $arg_value;
}