PHP Array_replace argument not an array?

I'm modifying Wordpress post formats and displaying them in a custom tab bar horizontally across the top of the Wordpress Post Editor page.

I'm trying to rename the labels in the array before they get printed.

Var_dump($format_string) outputs:

Using array_replace I try to change the labels:

$replacements = array(1 => "Code", 2 => "External", 3 => "Swap", 4 => "Horizontal", 5 => "Single" );
$post_types = array_replace($format_string, $replacements);

echo '<li>' . esc_html($post_types) . '</li>';
}

but the error is:

Warning: array_replace(): Argument #1 is not an array in ... on line 19

Full code:

foreach ($post_formats as $format) {
    $class = ($format == $current_post_format || (empty($current_post_format) && $format == 'standard') ? 'current' : '');
    if ($format == 'standard') {
        $format_string = __('Standard', 'post-format');
        $format_hash = 'post-format-0';
    } else {
        $format_string = get_post_format_string($format);
        $format_hash = 'post-format-'.$format;
    }
    var_dump($format_string);

    $replacements = array(1 => "Code", 2 => "External", 3 => "Swap", 4 => "Horizontal", 5 => "Single" );
    $post_types = array_replace($format_string, $replacements);

    echo '<li><a '.(!empty($class) ? 'class="'.esc_attr($class).'"' : '').' href="#'.esc_attr($format_hash).'">'.esc_html($post_types).'</a></li>';
}