changing array to a string

When I submit the for below it gives me an array that looks like this "array(5) { [0]=> string(1) "3" [1]=> string(1) "4" [2]=> string(1) "5" [3]=> string(1) "6" [4]=> string(1) "7" } " I would like to convert it to a string like this "3,4,5,6,7" <form action="test.php" method="post"> <table> <tr><td colspan="2" align="left"><input name="subscriptions[]" value="3" type="checkbox">test message </td></tr><tr><td colspan="2" align="left"><input name="subscriptions[]" value="4" type="checkbox">test message </td></tr><tr><td colspan="2" align="left"><input name="subscriptions[]" value="5" type="checkbox">test message </td></tr><tr><td colspan="2" align="left"><input name="subscriptions[]" value="6" type="checkbox">test message </td></tr><tr><td colspan="2" align="left"><input name="subscriptions[]" value="7" type="checkbox">test message </td></tr> </table> <input type="hidden" name="doit" value="yes"> <input type="submit" name="submit"> </form>