the script:

<?php
echo "A: ", (double)microtime()*1000000;
setlocale(LC_NUMERIC,"pl");
echo "<BR>";
echo "B: ", (double)microtime()*1000000;
?>

the script's output:

A: 773149
B: 0

it works however under Solaris 2.6:

A: 588313
B: 590748

It is possible pl locale for Solaris 2.4 is somehow broken (although it seems elsewhere to work). Nevertheless even buggy locale shouldn't prevent other (PHP) functions (in consequence it breaks mt_srand and mt_rand) from working.

What is interesting: all locale categories do not produce this effect except LC_NUMERIC and LC_ALL.