<?php

$tableau = array(
    "jean" => 14,
    "claude" => 13,
    "david" => 18,
    "maxime" => 1,
);

$moyenne = 0;

foreach ($tableau as $prenom => $note) {
    echo "$prenom: $note<br/>";
    $moyenne += $note;
}

$moyenne = $moyenne / count($tableau);

echo "<p><strong>Moyenne :</strong> $moyenne";

// Modification des notes 
foreach ($tableau as $prenom => $note) {
    if ($prenom == "jean") {
        continue;
    }
    $tableau[$prenom] = $note - 2;
}

echo "<hr><h3>Nouvelles notes</h3>";

?>

<pre>

<?php
print_r($tableau);
?>

</pre>