$x = $_POST['x'];
$y = $_POST['y'];
$geom="ST_GeomFromText('POINT($x $y)',2154)";
$r = $bdd->prepare("INSERT INTO ouvragesp (id, yl93, xl93,annee,geom) VALUES(:identifiant , :y , :x , 2017, :geom)");
$r->execute(array(
  'identifiant' => $id,
  'x' => $x,
  'y' => $y,
  'geom' => $geom));

PDOStatement::execute(): SQLSTATE[XX000]: Internal error: 7 ERROR: parse error - invalid geometry HINT: "ST" <-- parse error at position 2 within geometry

$r = $bdd->prepare("INSERT INTO ouvragesp (id, yl93, xl93,annee,geom) VALUES(:identifiant , :y , :x , 2017,ST_GeomFromText(:geom,2154))");
  $r->execute(array(
  'identifiant' => $id,
  'x' => $x,
  'y' => $y,
  'geom' => "POINT($x $y)"));