Trying to concatenate using a string variable and text

In the simplest form I need to built a query using a variable concatenated with some text. Something like this:
$query = "select * from table where fieldname $oper '$key[0]' "

And then I'd like to concatenate that $query with the following in a while loop:
$query = $query || " $logic fieldname $oper '$key[$i]' "

and when the while loop is done concatenate one last time with the following:
$query = $query || "order by fieldname"

$oper has been set to the regular expression characters ~ or ~* for case sensitivity
$logic has been set to OR

This code worked under PHP2 but I can't seem to get it to work under PHP3. I thought that all I would have to do is replace the plus sign (which was the concatenation character in PHP2 with the || which is the concatenation character in PHP3. I am new to PHP and any help I can get is greatly appreciated.

Sue