Syntaxfehler: ALTER TABLE...

[linksetzer]

http://www.php.net/manual/en/language.types.string.php#language.types.string.parsing.complex

also statt

"ALTER TABLE "$tablename" CHANGE "$old[$k]" "$neu[$k]" TEXT DEFAULT NULL;"

so

"ALTER TABLE $tablename CHANGE '{$old[$k]}' '{$neu[$k]}' TEXT DEFAULT NULL;"