@moostende at gmail dot com
If you want to remove all dashes but one from the string '-aaa----b-c-----d--e---f' resulting in '-aaa-b-c-d-e-f', you CAN use str_replace !

<?php
function foo($str)
{
    do {
        $str = str_replace("--", "-", $str, $count);
    } while ($count > 0);
    return $str;
}
echo foo("-aaa----b-c-----d--e---f");
?>

This outputs the following: 
-aaa-b-c-d-e-f