Looks like this function has a strange randomness.
If you take any number of elements in an array which has 40..100 elements, the 31st one is always by far the less occuring (by about 10% less than others).
I tried this piece of code at home (PHP Version 5.3.2-1ubuntu4.9) and on my server (PHP Version 5.2.17), unfortunately i haven't any server with the last version here :
$valeurs = range(1, 40);
$proba = array_fill(1, 40, 0);
for ($i = 0; $i < 10000; ++$i)
$tirage_tab = array_rand($valeurs, 10);
foreach($tirage_tab as $key => $value)
echo "Proba : <br/>\n";
foreach($proba as $key => $value)
echo "$key : $value<br/>\n";
In every try, the number of occurrences change a bit but the 31 is always far less (around 2200) than the others (2400-2600). I tried with 50 and 100 elements, no change. I tried with more or less elements to pick (second parameter to array_rand), same result. If you pick only one element it's even worse : 31 has half the result of the others.
For this particular case, i recommend shuffling the array and taking the nth first elements, in this test it's 60% faster and the statistics are ok.