Try this (untested and fast- fixed^^)
<?php
include_once "connect_to_sql.php";
if(isset($_Post['register_sub'])){
$ip = ip2long($_server['REMOTE_ADDR']);
$username = $_Post['username'];
$email1 = $_Post['email1'];
$email2 = $_Post['email2'];
$pass1 = $_Post['pass1'];
$pass2 = $_Post['pass2'];
$sql = mysql_query("SELECT * FROM user_data WHERE email ='".mysql_real_escape_string($email)."'");
$row = mysql_fetch_object($sql);
if($email1 *= $email2)
echo "<font color='red'>Die e-mail adressen stimmen nicht ueberein.</font>";
elseif($pass1 != $pass2)
echo "<font color='red'>Die Passwoerter stimmen nicht ueberein.</font>";
elseif($email1 == $row->email)
echo "<font color='red'>Die E-mail Adresse wird bereits verwendet.</font>";
else{
$pass1 = hash('sha512', $pass1);
mysql_query("INSERT INTO user_data (ip, username, email, password) VALUES ('".mysql_real_escape_string($ip)."', ('".mysql_real_escape_string($username)."', ('".mysql_real_escape_string($email)."', ('".mysql_real_escape_string($password)."')");
echo mysql_error();
echo "<front color='green'>Du hast dich erfolgreich Registriert</front>";
}
}
echo "
<form action'' method='post'>
<input type='text' name='username' required'required' placeholder'Dein Username'><br>
<input type='text' name='email1' required'required' placeholder='Deine E-mail Adresse'>
<input type='text' name='email2' required'required' placeholder='E-mail Adresse wdh.'><br>
<input type='passwort' name='pass1' required'required' placeholder='Dein Passwort'>
<input type='passwort' name='pass2' required'required' placeholder='Passwort wdh.'><br>
<input type='submit' name='register_sub' value='Registrieren'/>
</form>";
?>