too long

I have a function to calculate some mathmatic, but it seems like it wont put into database...

Is there something i dont know or?

function overskud($odds, $indskud) {
    $overskud1 = $indskud * $odds;
    $overskud2 = $overskud1 - $indskud;
    echo "$overskud2";
}   
$overskud = overskud($odds, $indskud);
mysql_query("INSERT INTO ODDS_kupon (kamp,odds,indskud,overskud,resultat,af,dato_ar,dato_moned,dato_dag) VALUES ('$kamp', '$odds', '$indskud', '$overskud2', '0', '$profilid', '$datoar', '$datomoned', '$datodag')");

Your query uses the variable $overskud2, but you did not define it (it only exists inside the function). I suppose you meant $overskud.

And the function should return its result, not echo it (confused with shell scripting?).

You need to return the result rather than echo it:

function overskud($odds, $indskud) {
    $overskud1 = $indskud * $odds;
    $overskud2 = $overskud1 - $indskud;
    return "$overskud2";
}   
$overskud = overskud($odds, $indskud);
mysql_query("INSERT INTO ODDS_kupon (kamp,odds,indskud,overskud,resultat,af,dato_ar,dato_moned,dato_dag) VALUES ('$kamp', '$odds', '$indskud', '$overskud2', '0', '$profilid', '$datoar', '$datomoned', '$datodag')");

You may need to alter this to return an array of values that includes $overskud2 in addition to $overskud, since your INSERT query uses that variable (but unless it is defined elsewhere in code not posted, it is not in scope due to being inside the function).

echo simply outputs data to the page/buffer, return actually passes the value back to whatever invoked it and allows for variable assignment via function output.

betr try this.. you should be returning something to the function call

function overskud($odds, $indskud) {
    $overskud1 = $indskud * $odds;
    $overskud2 = $overskud1 - $indskud;
    return $overskud2;
}   
$overskud = overskud($odds, $indskud);
mysql_query("INSERT INTO ODDS_kupon(kamp,odds,indskud,overskud,resultat,af,dato_ar,dato_moned,dato_dag) VALUES('$kamp','$odds', '$indskud', '$overskud', '0', '$profilid', '$datoar', '$datomoned', '$datodag')");