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mysql_query("INSERT INTO tabelle (abc, def)
VALUES ('$var_abc', '$var_def')");

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mysql_query("INSERT INTO tabelle (abc, def)
VALUES ('".$var_abc."', '".$var_def."')");

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mysql_query("INSERT INTO tabelle (abc, def)
VALUES ('".addslashes($var_abc)."', '".addslashes($var_def)."')");