Trying to use variable in MySQL INNER JOIN - php

Question

I am a php and MySQL newbie. What I have done is created an html form with a <select> dropdown. Based on the selection from the form, it changes the $_SESSION[campaignID] variable. Changing the selection in the form is supposed to then change what displays on the page. The page consists of a forum style post that allows users to fill out a textarea and then submit it into the MySQL database into a table called "posts." On this same page I then display the contents of the "posts" table.
What I have done is in the posts MySQL table, I have added a campaignID row. Then in my campaigns table I also have a campaignID row. The campaignID in campaigns table auto increments every time a campaign is created. Then with the earlier mentioned dropdown I can select the campaign I want, and it should then show all posts with the same campaignID as the campaign I selected.
I can verify that the $_SESSION[campaignID] is changing when i select the various options. Because when I do that and then save another post, it takes the session variable campaignID and saves it properly in the posts table.
Now what I need it to do, is when I change the drop down (which then changes $_SESSION[campaignID]) I need it to display the posts that share the same campaignID as the selected campaign. I am just not able to get it to display when trying to put a variable in my INNER JOIN query. I have a feeling the info I have provided may not be enough, but not sure what else I need to include here. Help?
Code that contains the INNER JOIN query and displays the various rows of posts table
UPDATED
<?php
$con=mysqli_connect("localhost","dorians","ds2953!b67P$","aldentec");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$campaignID = $_SESSION['campaignID'];
$result = mysqli_query($con,"SELECT posts.postDate, posts.postName, posts.postEntry FROM posts INNER JOIN campaigns ON posts.campaignID=" . $campaignID);
while($row = mysqli_fetch_array($result))
{
echo "<div id='campaignPostContainer'>";
echo "<ul class='campaignPostBox'>";
echo "<p class='postInfo'>";
echo "Posted on:";
echo "<li>" . $row['postDate'] . "</li>";
echo "</p>";
echo "<p class='postInfo'>";
echo "Posted by:";
echo "<li>" . $row['postName'] . "</li>";
echo "</p>";
echo "<li class='postEntry'>" . $row['postEntry'] . "</li>";
echo "</ul>";
echo "</div>";
echo "<hr>";
}
mysqli_close($con);
?>

Answer 1

Without going into a use-prepared-statements rant...
You're not pulling the session variable, but assigning $campaignId to literally the string $_SESSION[campaignID].
Change your line:
$campaignID = '$_SESSION[campaignID]';
to:
$campaignID = $_SESSION['campaignID'];
Also, your query is going to generate a cross product unless you define something in your ON clause like:
SELECT posts.postDate, posts.postName, posts.postEntry FROM posts
INNER JOIN campaigns ON posts.campaignID= $campaignID
AND posts.campaignID= campaigns.id

Answer 2

The value should be in single quote ' so you may try this
posts.campaignID='" . $campaignID ."'"
instead of
posts.campaignID=" . $campaignID