Note that this does not replace strings that become part of replacement strings. This may be a problem when you want to remove multiple instances of the same repetative pattern, several times in a row.

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 cannot use str_replace. Instead, use preg_replace:

<?php
$challenge = '-aaa----b-c-----d--e---f';
echo str_replace('--', '-', $challenge).'<br>';
echo preg_replace('/--+/', '-', $challenge).'<br>';
?>

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