与任何事情一样，如果你能把它分解成更小的部分，一次解决一个问题，它将帮助你找到问题的根源。
我浏览了你的代码，把我的评论放在里面。除了一些缺少的牙套，还有一个 </p> 以及潜在的sql注入漏洞，您认为它不应该做什么？您应该调试和/或将查询打印到日志文件以查看它们。

$control = 0;
        $abfrage = "SELECT * FROM youtuber WHERE username = '$user' AND url = '$url'";
        // as other answers have pointed out, this kind of query 
        // is a really bad idea. Think of what happens if $user 
        // is something like the following: 
        // ' or true; drop table youtuber; --

        $ergebnis = mysqli_query($verbindung,$abfrage);
        while ($row = mysqli_fetch_object($ergebnis)) {
            $control++;
        }

        if ($control != 0) {
            // if the control variable was incremented (a user exists)
            echo "<p class='text' style='color: red;'>Dieser Youtuber ist bereits eingetragen!"; // here you don't close your paragraph element
        } else {
            // no user was found, create
            $eintrag = "INSERT INTO youtuber (username,url) VALUES ('$user','$url')";
            $eintragen = mysqli_query($verbindung,$eintrag);
        }
        if ($eintragen == true){
            // the insert query was successful
            echo "<p>Youtuber erfolgreich eingetragen du wirst nun weitergeleitet!</p>";
        } else {
            // the insert query failed
            echo "<p class='text' style='color: red;'>Fehler: System: bitte Kontaktiere einen Administrator!</p>";
        }

        mysqli_close($verbindung);

// trailing braces -- what are these?
    }
}