In addition to what @Niet Suggested you have to know that you cannot reuse the same name for the placeholder like you did:
INSERT INTO veranstaltung_anfrage (....) VALUES (..:a, :a, :a, :a, :a, :a, :a)
You will need to give them unique names:
INSERT INTO veranstaltung_anfrage (....) VALUES (..:a1, :a2, :a3, :a4, :a5....)
then bind them one by one
0
solved PDO in PHP doesn´t work – no error