Cool signature script

This is a quick fun script for people wanting to have a cool signature of different forums. This script will let users add new messages and display them on your signature dynamically. Installation is very easy, just create a file with the following code and another file named 'data.txt' in the same folder. Don't forget to chmod data.txt to 0755.

<?php
$sigs = array();
$file = 'data.txt';
$contents = file_get_contents($file);
if(!empty($contents))
    $sigs = json_decode($contents);
if(isset($_GET['add'])){
    if(isset($_GET['clr'])){
        file_put_contents($file,'');
        $sigs = array();
    }
    if(!empty($_POST['submit'])){
        $nom = $_POST['nom'];
        $msg = $_POST['msg'];
        if(!empty($nom) && !empty($msg)){
            $nom = preg_replace('@[^a-z0-9 ><_\-\?\.]@i','',$nom);
            $msg = preg_replace('@[^a-z0-9 ><_\-\?\.]@i','',$msg);
            if(!(strlen($nom)<=12 && strlen($msg)<=70)){
                $nom = substr($nom,0,12);
                $msg = substr($msg,0,12);
            }
            $sigs[] = array('time'=>$_SERVER['REQUEST_TIME'],'nom'=>$nom,'msg'=>$msg);
        }
    }   
        {
        echo'
        <img src="?img" alt="" border="0" />
        <br />
        <form method="POST">
        <input type="hidden" value="" name="add" />
        <input type="text" value="" name="nom" size="12" />
        <input type="text" value="" name="msg" size="70" />
        <input type="submit" name="submit" value="Add" />
        </form>
        ';
        echo"<a href='?img'>view sig</a> | <a href='?add'>add msg</a> | <a href='?add&clr'>clear all</a>";
    }
$sigs = json_encode($sigs);
file_put_contents($file,$sigs);
}
elseif(isset($_GET['img'])){
    if(!empty($sigs)){
        $sigs = array_reverse($sigs);
        $img = imagecreatetruecolor(800,120);
        imagecolorallocate($img,0,0,0);
        $color = imagecolorallocate($img,255,255,255);
        $i = 0;
        foreach($sigs as $sig){
            if($i>=5)
                break;
            imagestring($img,15,4,10+($i*20),'| '.$sig->nom.' : '.$sig->msg,$color);
            imagestring($img,15,720,10+($i*20),date('h:i:s',$sig->time),$color);
            
            $i++;
        }
        header('Content-type: image/png');
        imagepng($img);
        imagedestroy($img);
    }
}else{
    echo"<a href='?img'>view sig</a> | <a href='?add'>add msg</a> | <a href='?add&clr'>clear all</a>";
}
?>