I just tried it, for me the first one was consistently slower by around 0.0005 seconds over 10,000 iterations. You can use this bit of code to quickly benchmark code, run the script a few times to get a consistent time.
$start = explode(' ', microtime());
$start = $start['1'] + $start['0'];
$i = 0;
// Change 10000 to the number of times to run the code. The larger the function the less you will likely want to run it.
while ($i != 10000)
// Code you want to benchmark goes here.
$finish = explode(' ', microtime());
$finish = $finish['1'] + $finish['0'];
echo '<p style="margin:auto;text-align:center">'.round(($finish-$start), 6).' seconds</p>';