PHP: trouble with understanding POST and GET at the same time - php

Question

I have a webpage where I ask input from the user and when he clicks the add button, it should insert the input from the user into the database. I always thought that you couldn't use POST and GET at the same time but according to this and this answers it should be possible. Yet, I can't seem to get their solutions working.
The script inserts something in the db (an id is generated) but the seriesName field remains empty.
The PHP script:
<?php
$con=mysqli_connect("localhost","user","passwd","database");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$issueSeries = mysqli_real_escape_string($con, $_GET['addSeries']);
mysqli_query($con,"INSERT INTO series (seriesName) VALUES ('$issueSeries')");
mysqli_close($con);
?>
The HTML:
<div id="issueAddInformationLayout">
<div id="issueAddCredits">
<form action="addIssue.php" method="post">
<table>
<tr id="lblAddCreator">
<td><p>NR</p></td>
<td><p>Series</p></td>
<td><p>Volume</p></td>
<td><p>Title</p></td>
<td><p>Publisher</p></td>
</tr>
<tr>
<td><input type="text" id="addNR" size="3%"/></td>
<td><input type="text" id="addSeries" size="25%" /></td>
<td><input type="text" id="addVolume" size="25%" /></td>
<td><input type="text" id="addTitle" size="30%" /></td>
<td><input type="text" id="addPublisher" size="30%" /></td>
</tr>
</table>
<table>
<tr id="lblAddCreator">
<td><p>Writer</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddCreator">
<td><input type="text" id="addWriter1" size="30%" /></td>
<td><input type="text" id="addWriter2" size="30%" /></td>
<td><input type="text" id="addWriter3" size="30%"/></td>
<td><input type="text" id="addWriter4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Editor</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddEditor">
<td><input type="text" id="addEditor1" size="30%" /></td>
<td><input type="text" id="addEditor2" size="30%"/></td>
<td><input type="text" id="addEditor3" size="30%"/></td>
<td><input type="text" id="addEditor4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Letterer</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddLetterer">
<td><input type="text" id="addLetterer1" size="30%"/></td>
<td><input type="text" id="addLetterer2" size="30%"/></td>
<td><input type="text" id="addLetterer3" size="30%"/></td>
<td><input type="text" id="addLetterer4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Colourist</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddColourist">
<td><input type="text" id="addColourist1" size="30%"/></td>
<td><input type="text" id="addColourist2" size="30%"/></td>
<td><input type="text" id="addColourist3" size="30%"/></td>
<td><input type="text" id="addColourist4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Cover Artist</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddCoverArtist">
<td><input type="text" id="addCoverArtist1" size="30%"/></td>
<td><input type="text" id="addCoverArtist2" size="30%"/></td>
<td><input type="text" id="addCoverArtist3" size="30%"/></td>
<td><input type="text" id="addCoverArtist4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Inker</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddInker">
<td><input type="text" id="addInker1" size="30%"/></td>
<td><input type="text" id="addInker2" size="30%"/></td>
<td><input type="text" id="addInker3" size="30%"/></td>
<td><input type="text" id="addInker4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Penciler</p></td>
<td></td>
<td></td>
</tr>
<tr id="txtAddPenciler">
<td><input type="text" id="addPenciler1" size="30%"/></td>
<td><input type="text" id="addPenciler2" size="30%"/></td>
<td><input type="text" id="addPenciler3" size="30%"/></td>
<td><input type="text" id="addPenciler4" size="30%"/></td>
</tr>
<tr id="lblAddCreator">
<td><p>Trade Paper Back</p></td>
<td><p id="lblAddCover">Cover</p></td>
</tr>
</table>
<table>
<tr id="txtAddTPB">
<td><input type="text" id="addTPB" size="30%"/></td>
<td>
<p id="btnAddCover" enctype="multipart/form-data" action="parse_file.php" method="post">
<input type="file" name="uploaded_file">
</p>
</td>
<td>
<id="btnAddIssue"><input type="submit" value="Add">
</td>
</tr>
</table>
</form>
</div>
</div>
There seemed to be some confusion about the HTML so I added the full HTML code. Keep in mind: this is purely for myself and I don't really care about stuff like that tables are outdated :)

Answer 1

In HTTP, you can't use GET and POST at the same time but you can make a POST request which has a query string as part of the URL.
PHP will populate $_POST with the body of the request and $_GET with the query string, even if the request was a POST request.
Since the form is POST, the data from the inputs will be put into the body and not the query string (so will appear in $_POST not $_GET).
If the value of addSeries was fixed then you could use it with $_GET like so:
<form id="btnAddIssue" action="addIssue.php?addSeries=someValue" method="post">
… but since you are taking user input, use $_POST to read the value after you put the input inside the form.

Answer 2

Your input is outside of your form so it isn't submitted.
<form id="btnAddIssue" action="addIssue.php" method="post">
<input type="text" id="addSeries" size="25%" />
<input type="submit" value="Add">
</form>
You then need to change $_GET['addSeries'] to $_POST['addSeries'] since your form is set to POST (or change your form to submit via GET).

Answer 3

Your input is outside the form tag which won't be considered when user submits the form.
<form id="btnAddIssue" action="addIssue.php" method="post">
<input type="text" id="addSeries" size="25%" />
<input type="submit" value="Add">
</form>
And since you are making a post request onto php side it would be $_POST to reference that input's value
Also, sometimes it would be good to make use of $_REQUEST which contains data from both GET and POST so you have proper data from request everytime
$issueSeries = mysqli_real_escape_string($con, $_REQUEST['addSeries']);

Answer 4

Your text filed is placed outside the form and so no value (of that text filed) will be submitted to the action page.
Change it to
<form id="btnAddIssue" action="addIssue.php" method="post">
<input type="text" id="addSeries" size="25%" />
<input type="submit" value="Add">
</form>
Also use $_POST, or you can even use $_REQUEST (you can use $_REQUEST for both get and post variables)