Remplir champ

value="..."

<input type="text" name="champ3" id="champ3" value="valeurcalculée">

<?php

//récupération PROPRE des variables AVANT de les utiliser
$champ1 = !empty($_POST['champ1']) ? $_POST['champ1'] : NULL;
$champ2 = !empty($_POST['champ2']) ? $_POST['champ2'] : NULL;
$champ3 = !empty($_POST['champ3']) ? $_POST['champ3'] : '';

//traitement du formulaire si champ1 et champ2 sont remplis...
if($champ1 && $champ2){
  $champ3 = $champ1 + $champ2;
}

?>
<!-- formulaire -->
<form method="post" action="">
  <p>
  <label for="champ1">Votre Champ1 :</label>
  <input type="text" name="champ1" id="champ1" value="<?php echo $champ1;?>" required>

  <label for="champ2">Votre Champ2 :</label>
  <input type="text" name="champ2" id="champ2" value="<?php echo $champ2;?>" required>

  <label for="champ3">Votre Champ3 :</label>
  <input type="text" name="champ3" id="champ3" value="<?php echo $champ3;?>">
  </p>

  <input type="submit" value="Envoyer" />
</form>