PHP index is not recognized (post method) - php

Question

I am very new to php and am still in the learning phase. I typed a small script 1 to 1 and had to find out that the index from my HTML file is not recognized in php. In the given video the "GET" method is used, but it should actually work exactly the same way. It is just a test script. Nevertheless, an answer would be helpful.
Here are my scripts:
HTML:
<form id="register-form" action="Memeon.php" method="post">
<img class="register_avatar" src="pictures/login_register_avatar.png" oncontextmenu="return false;">
<h2>Welcome to Memeon</h2>
<div class="reg-input-div one">
<div class="i">
<i class="fas fa-user"></i>
</div>
<div>
<h5>Username</h5>
<input id="reg_un" class="reg-input" name="username" type="text" autocorrect="off" autocapitalize="off" spellcheck="false">
</div>
</div>
<div class="reg-input-div two">
<div class="i">
<i class="fas fa-at"></i>
</div>
<div>
<h5>Email-Adress</h5>
<input id="reg_ea" class="reg-input" name="email" type="email" autocorrect="off" autocapitalize="off" spellcheck="false">
</div>
</div>
<div class="reg-input-div three">
<div class="i">
<i class="fas fa-lock"></i>
</div>
<div>
<h5>Password</h5>
<input id="reg_pw" class="reg-input" name="password" type="password" autocorrect="off" autocapitalize="off" spellcheck="false">
</div>
</div>
<a id="rl_l1" href="#">2-Factor-Authentication</a>
<a id="rl_l2" href="#">Forgot Password?</a>
<input type="submit" onclick="setRegisterInfo()" class="register_btn" value="Sign in">
</form>
PHP:
<?php
echo "Hallo ". $_POST["username"];
?><br>
Der sehnlichst erwünschte Newsletter wird gesendet an <?php echo $_POST["email"]; ?>
Both are in the same folder. With a submit button I get to the PHP page. An error always appears there.

Answer 1

Maybe it haves to be with something in your "setRegisterInfo()" function (is it implemented ? if so, you should post it here too :) ) in the "onclick" attribute of the submit input at the very end. If that function breaks, the form won't be submitted.
<input type="submit" onclick="setRegisterInfo()" class="register_btn" value="Sign in">
Try again without that function.
<input type="submit" class="register_btn" value="Sign in">