PHP session variable not being stored

Solution:

Try to check if$_POST['libelle'] exists too.
Also try to post to the same page by empty theaction of your form.
This will automatically be the same page when empty.

<?php
session_start();
require "header.php";
?>

  <form action="" method="post">
    <div >
      <label for="libelle">Libelle : </label>
      <input type="text" name="libelle" id="libelle" required value="<?php echo isset($_SESSION['libelle']) ? $_SESSION['libelle'] : "" ?>">
    </div>
    <button type="submit" name="insert_secteur" id="insert_secteur">Insert secteur</button>
  </form>

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST' AND isset($_POST['insert_secteur']) AND isset($_POST['libelle'])) {
  $_SESSION['libelle'] = $_POST['libelle'];
}
require "footer.php";
?>
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Solution:

It does work if you remove action url (If you post to same page):

<?php
  session_start();
  require "header.php";
  ?>

    <form action="" method="post">
      <div >
        <label for="libelle">Libelle : </label>
        <input type="text" name="libelle" id="libelle" required value="<?php echo isset($_SESSION['libelle']) ? $_SESSION['libelle'] : "" ?>">
      </div>
      <button type="submit" name="insert_secteur" id="insert_secteur">Insert secteur</button>
    </form>

  <?php
  if (isset($_POST['libelle'])) {
    $_SESSION['libelle'] = $_POST['libelle'];
  }


  echo $_SESSION['libelle'];
  require "footer.php";
?>
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If you post to other page then post execution must be done on other page.

But if you need to know specific form was sent you can do this: By saving hidden input with key name

<?php
  session_start();
  require "header.php";
  ?>

    <form action="" method="post">
      <div >
        <label for="libelle">Libelle : </label>
        <input type="text" name="libelle" id="libelle" required value="<?php echo isset($_SESSION['libelle']) ? $_SESSION['libelle'] : "" ?>">
        <input type="hidden" name="source" value="createSecteur"/>
      </div>
      <button type="submit" name="insert_secteur" id="insert_secteur">Insert secteur</button>
    </form>

  <?php
  if($_SERVER["REQUEST_METHOD"] == "POST"){
    if (empty($_POST["source"])){$source = null;}else{$source = htmlentities($_POST['source'], ENT_QUOTES, "UTF-8");}

    if($source=='createSecteur'){
      if (isset($_POST['libelle'])) {

        $_SESSION['libelle'] = $_POST['libelle'];
      }
    }
  }


  echo $_SESSION['libelle'];
  require "footer.php";
?>
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