Parser

<form action="" method="post">
    <textarea name="parse"></textarea><br>
    <input type="submit" value="run your code!">
</form>

<br><br><br><br>

<?php

ini_set('display_errors',1);
ini_set('display_startup_errors',1);
error_reporting(-1);

if(isset($_POST) && isset($_POST["parse"]))
    echo parse($_POST["parse"]);
else
echo parse("If 10 == 10, then here will appear a <<IF [10] == [10] FI>>, but there is also the possibility to get a <<IF [30] < [10] FI>>,<br>
    therefor the IF clause I wrote there have to be wrong, but that is not the case.<br> But it can also happen a error like this: <<IF[30]=[30]FI>><br>
    soon there will be the possibility to add variables, they will be used like this:<br>
    define: <<[HalloWelt]=[13]>><br>
    and output: <<ECHO[HalloWelt]ECHO>><br>
    Now a litte test: <<IF[<<IF[10]==[10]FI>>]==[0]FI>>");

function parse($toParse) {
    $variables = [];

    if(preg_match_all("/<<+(.*?)>>/", $toParse, $matches)) {
    foreach($matches[1] as $match) {
            $save = $match;

            $match = str_replace(" ", "", $match);

            if(preg_match_all("/if+(.*?)fi/i", $match, $ifs)) {
                foreach($ifs[1] as $if) {
                    $if = parse($if);

                    $if = str_replace(">", "", $if, $gt);
                    $if = str_replace("<", "", $if, $lt);
                    $if = str_replace("==", "", $if, $eq);

                    preg_match_all("/\[+(.*?)\]/", $if, $params);
                    if(count($params[1]) > 2 || count($params[1]) < 2) die ("Syntax Error!");

                    if($gt == 1 && $lt != 1 && $eq != 1) {
                        if($params[1][0] > $params[1][1])
                            $toParse = str_replace("<<".$save.">>","1", $toParse);
                        else
                            $toParse = str_replace("<<".$save.">>","0", $toParse);
                    } else if($gt != 1 && $lt == 1 && $eq != 1) {
                        if($params[1][0] < $params[1][1])
                            $toParse = str_replace("<<".$save.">>","1", $toParse);
                        else
                            $toParse = str_replace("<<".$save.">>","0", $toParse);
                    } else if($gt != 1 && $lt != 1 && $eq == 1) {
                        if($params[1][0] == $params[1][1])
                            $toParse = str_replace("<<".$save.">>","1", $toParse);
                        else
                            $toParse = str_replace("<<".$save.">>","0", $toParse);
                    } else {
                        $toParse = str_replace("<<".$save.">>","Syntax Error!", $toParse);
                    }
                }
            }
            else if(preg_match_all("/echo+(.*?)echo/i", $match, $echos)) {
                foreach($echos[1] as $echo) {
                    preg_match_all("/\[+(.*?)\]/", $echo, $params);
                    if(count($params[1]) == 1)
                        $toParse = str_replace("<<".$save.">>",$variables[strtolower($params[1][0])], $toParse);
                    else
                        $toParse = str_replace("<<".$save.">>","Syntax Error!", $toParse);
                }
            } else {
                $match = str_replace("=", "", $match, $ct);
                if($ct < 1 || $ct > 1)
                    $toParse = str_replace("<<".$save.">>","Syntax Error!", $toParse);
                else {
                    preg_match_all("/\[+(.*?)\]/", $match, $params);

                    if(count($params[1]) == 2) {
                        $variables[strtolower($params[1][0])] = $params[1][1];
                        $toParse = str_replace("<<".$save.">>","", $toParse);
                    } else
                        $toParse = str_replace("<<".$save.">>","Syntax Error!", $toParse);
                }
            }
        }
    }
    return $toParse;
}

?>