Creating custom markup using PHP

markup.php

<?php
    function string_to_html(string $str) {
        $str = str_ireplace("[newline]", "<br/>", $str);
        $str = str_ireplace("[bold]", "<b>", $str);
        $str = str_ireplace("[/bold]", "</b>", $str);
        $str = str_ireplace("[italic]", "<i>", $str);
        $str = str_ireplace("[/italic]", "</i>", $str);
        $str = str_ireplace("[center]", "<span style=\"text-align:center;\">", $str);
        $str = str_ireplace("[/center]", "</span>", $str);
        $str = str_ireplace("[left]", "<span style=\"text-align:left;\">", $str);
        $str = str_ireplace("[/left]", "</span>", $str);
        $str = str_ireplace("[right]", "<span style=\"text-align:right;\">", $str);
        $str = str_ireplace("[/right]", "</span>", $str);
        $str = str_ireplace("[header]", "<span style=\"text-align:left;\">", $str);
        $str = str_ireplace("[/left]", "</span>", $str);
        $str = str_ireplace(" ", "&nbsp;", $str);
        
        return $str;
    }

    echo string_to_html("[bold]Hello       
                        World[/bold]");
?>