How do I get my $_POST to recognize the values - php

Question

$_POST won't recognize the value mailuid from the login form on this page or others (profile page).
$_Get methods do not work because of how the login system is built and unsecured.I need mailuid value to bring them to their own profiles page after login.
Login Form since its's post method I should be able to grab the value on other pages and this one
<div class="modal">
<div class = "modal-content">
<section class="section-default">
<h1>Login</h1>
<?php
if (!isset($_SESSION['Id'])) {
echo'<form action="includes/login.inc.php" method="post">
<input type="text" name="mailuid" placeholder="Username/E-mail...">
<input type="password" name="pwd" placeholder="Password...">
<button type="submit" name="login-submit">Login</button>
</form>';
} else if (isset($_SESSION['Id'])) {
echo '<div class="signup12">
You Do not have an account? Sign Up
</div>
<div class="forgotpwd">
Forgot your password?
</div>';
}
?>
</section>
</div>
</div>
Temporary check for the mailuid value. Supposed to grab the value form the login form a spit it back out, to check to see if it is recognized
<?php
$user = $_POST["mailuid"];
if (isset($_POST["mailuid"]))
{
$user = $_POST["mailuid"];
echo $user;
echo " is your username";
}
else
{
$user = null;
echo "no username supplied";
}
?>

Answer 1

First I would clean this up:
$user = $_POST["mailuid"];
if (isset($_POST["mailuid"]))
{
$user = $_POST["mailuid"];
echo $user;
echo " is your username";
}
else
{
$user = null;
echo "no username supplied";
}
Instead it can be written more concise:
$user = isset($_POST["mailuid"]) ? $_POST["mailuid"] : false;
if( $user ){
echo "{$user} is your username";
} else {
echo "no username supplied";
}
I prefer Boolean false over NULL, null just means it doesn't exist. Boolean false lets you know you checked it and it didn't exist. Generally should should access $_POST as few times as you can. This is because you should never trust $_POST.
$_Get methods do not work because of how the login system is built and unsecured.
Post is no more secure than get, it's quite easy to post anything to the page even without visiting the site by using something like PostMan etc. Once you assign it to a local variable you know you have at least normalized the data, even if you haven't sanitized it yet.
Also don't forget to call session_start before trying to access $_SESSION. Because of the vagueness of the question, it could be that the form works fine, just the session data isn't being maintained because you haven't started the session yet.. etc....
Hope it helps.
Personally I would clean up the HTML part that makes the form as well, so instead of this:
<div class="modal">
<div class = "modal-content">
<section class="section-default">
<h1>Login</h1>
<?php
if (!isset($_SESSION['Id'])) {
echo'<form action="includes/login.inc.php" method="post">
<input type="text" name="mailuid" placeholder="Username/E-mail...">
<input type="password" name="pwd" placeholder="Password...">
<button type="submit" name="login-submit">Login</button>
</form>';
} else if (isset($_SESSION['Id'])) {
echo '<div class="signup12">
You Do not have an account? Sign Up
</div>
<div class="forgotpwd">
Forgot your password?
</div>';
}
?>
</section>
</div>
</div>
I would do something like this:
<div class="modal">
<div class = "modal-content">
<section class="section-default">
<h1>Login</h1>
<?php if (!isset($_SESSION['Id'])){ ?>
<form action="includes/login.inc.php" method="post">
<input type="text" name="mailuid" placeholder="Username/E-mail...">
<input type="password" name="pwd" placeholder="Password...">
<button type="submit" name="login-submit">Login</button>
</form>
<?php }else{ ?>
<div class="signup12">
You Do not have an account? Sign Up
</div>
<div class="forgotpwd">
Forgot your password?
</div>';
<?php } ?>
</section>
</div>
</div>
See how much cleaner that is. Most of this is just readability issues. For example there is no need to check if isset($_SESSION['Id']) in the else if condition, because it's either set or not. This is one less place to maintain the session variable key, and it makes the code less convoluted.
As for the actual problem, as long as you are reaching the above code after submission of the form, it should work. So that leads me to believe that you have something wrong in the action.
You should get a clean page after going to includes/login.inc.php meaning there shouldn't be much in the way of HTML. One thing you can do that is real simple is just add at the top:
die(__LINE__.' of '.__FILE__);
$user = isset($_POST["mailuid"]) ? $_POST["mailuid"] : false;
//... other code
What this will do is die which kills PHP execution, but outputs the argument you passed in. In this case I'm just putting the line and file that the die is on, that way it's easier to find later. But the point is to see if you are even hitting the correct ending script or the forms action/endpoint.
I only suggest this because you are really vague in what it's current behaviour is
$_POST won't recognize the value mailuid from the login form on this page or others (profile page).
For example, this doesn't tell me if you are even hitting the right page. Now had you said something like "all it does is output no username supplied". Then I would at lest know that. As I said above it could be just an omission of sesion_start() which must be called before attempting to access any $_SESSION stuff. You should call it only once, at the top of each page that uses sessions.
Although it's not a solution, it was too much to post in a comment. I would really like to help you more, but there just isn't enough information to go on.