你能发现我的PHP中的任何错误吗？ 网站给出500错误..？ [重复]

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Getting 500 Error when redirecting to this php file.

everything pretty much ive tried.

Not sure if it has anything to do with my hosting provider, which I HIGHLY doubt. i've checked through the sql and php and everything seems to be fine, been stuck on it for a good while.

include("GlobalVariables.php");

$conn=mysqli_connect('this info is correct');
$email=$_POST["EMAIL"];
$username=$_POST["USERNAME"];
$password=$_POST["PASSWORD"];
$ip=$_SERVER['REMOTE_ADDR'];
$date = date('Y-m-d');

if (check_username_valid($conn,$username)==false){
    $check1 = inject_sql($conn,"SELECT * FROM `userdata` WHERE `ip` = '$ip'");
    $check2 = inject_sql($conn,"SELECT * FROM `userdata` WHERE `username`='$username'");
    if (mysqli_row_count($check)==$max_accounts&&$check2["username"]==null){
        inject_sql($conn,"INSERT INTO `userdata`(`ip`, `username`, `password`, `email`, `reg_date`, `verified`) VALUES ('$ip','$username','$password','$email','$date',false");
        echo "0";
        $check3 = inject_sql($conn,"SELECT * FROM `userdata` WHERE `reg_date` = '$date' AND `username` = '$username");
        echo "1";
        if ($check3["username"] == "$username"){
            echo "2";
            header('Location:../index.html');
        }
    }
    else{
        header('Location:../index.html');
    }
}
else{

}

</div>