Why is my error message not echoing to the screen? - php

Question

When the user submits a form, the PHP will check to see if the $post was empty, if so, it will set an $error_message variable; thus database INSERT query will not execute. I then attempt to show the error message further on in the code, however it will not display. I have been trying to figure this out for hours and I still cannot find the solution. The funny thing is, this used to work, however I must have implemented something in this file which is preventing the code to execute correctly.
Why is my error message not echoing to the screen (last few lines of code show attempt to echo the error_message)?
<?php require("inc/db.php"); ?>
<?php include("inc/functions.php"); ?>
<?php include ("inc/ChromePhp.php"); ?>
<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
ini_set("html_errors", 1);
session_start();
if(!isset($_GET['id'])) {
header("Location: ?id=".getUserId($_SESSION['U_Email']));
}
if(isset($_SESSION["U_Email"])) {
$usersData = getUserInfo($_GET['id']);
$postCount = getPostCount($_GET['id']);
} else {
header('Location: login.php');
}
if($_SERVER["REQUEST_METHOD"] == "POST") {
$post = trim(filter_input(INPUT_POST,"user_post",FILTER_SANITIZE_SPECIAL_CHARS));
if($post == "") {
$error_message = "Please enter something before submitting";
}
if(!isset($error_message)) {
$post = $db->real_escape_string($post);
$postStore = $db->query("INSERT INTO `Post`(P_Body, P_Dateadded, User_U_ID) VALUES ('{$post}', NOW(), '{$_SESSION['U_ID']}' )");
}
}
$dashNav = true;
$cap = true;
$pageTitle = 'Profile';
$name = $usersData['U_Forename'] . " " . $usersData['U_Surname'];
$gender = $usersData['U_Gender'];
$bio = $usersData['U_Biography'];
$team = $usersData['U_Team'];
$city = $usersData['U_City'];
include("inc/header.php");
?>
<?php if(userExists($_GET['id'])) { ?>
<section>
<div class="wrapper">
<?php
if($_SESSION['U_ID'] != $_GET['id']) {
$testFollow = "SELECT `Following`.F_ID, `Following`.U_ID FROM `Following`
WHERE U_ID = '{$_SESSION['U_ID']}' AND F_ID = '{$_GET['id']}'";
$testFollowResult = $db->query($testFollow);
if ($testFollowResult->num_rows > 0) {
echo "<button class='lift' href='#'>Unfollow Driver</button>";
} else {
echo "<button class='lift' href='#'>Follow Driver</button>";
}
}
?>
<?php if (isset($error_message)) {
// This code will not echo!
echo "<h2>".$error_message."</h2>";
}
?>
</div>
<div class="section-b">
<div class="grid">
<div class="row">
<div class="col-wd-12">
<div class="col">
<form id="share" name="share" action="profile.php" method="post">
<textarea id="post" name="user_post" placeholder="What's happening?"> </textarea>
<span id="errorpost" class="error">You must input something something before sending</span>
<?php
echo "<div class='g-recaptcha' data- sitekey='6LfNTB0TAAAAAKEw9zfnFzvGCXF9MuYTkdB144x1
'></div>"; ?>
<button onclick="return postCheckValidate();" type="submit">Share</button>
</form>
</div>
</div>
</div>
</div>
</div>
<?php } ?>