If you're adding multiple values to an array in a loop, it's faster to use array_push than repeated [] = statements that I see all the time:

<?php
class timer
{
        private $start;
        private $end;

        public function timer()
        {
                $this->start = microtime(true);
        }

        public function Finish()
        {
                $this->end = microtime(true);
        }

        private function GetStart()
        {
                if (isset($this->start))
                        return $this->start;
                else
                        return false;
        }

        private function GetEnd()
        {
                if (isset($this->end))
                        return $this->end;
                else
                        return false;
        }

        public function GetDiff()
        {
                return $this->GetEnd() - $this->GetStart();
        }

        public function Reset()
        {
                $this->start = microtime(true);
        }

}

echo "Adding 100k elements to array with []\n\n";
$ta = array();
$test = new Timer();
for ($i = 0; $i < 100000; $i++)
{
        $ta[] = $i;
}
$test->Finish();
echo $test->GetDiff();

echo "\n\nAdding 100k elements to array with array_push\n\n";
$test->Reset();
for ($i = 0; $i < 100000; $i++)
{
        array_push($ta,$i);
}
$test->Finish();
echo $test->GetDiff();

echo "\n\nAdding 100k elements to array with [] 10 per iteration\n\n";
$test->Reset();
for ($i = 0; $i < 10000; $i++)
{
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
        $ta[] = $i;
}
$test->Finish();
echo $test->GetDiff();

echo "\n\nAdding 100k elements to array with array_push 10 per iteration\n\n";
$test->Reset();
for ($i = 0; $i < 10000; $i++)
{
        array_push($ta,$i,$i,$i,$i,$i,$i,$i,$i,$i,$i);
}
$test->Finish();
echo $test->GetDiff();
?>

Output

$ php5 arraypush.php
X-Powered-By: PHP/5.2.5
Content-type: text/html

Adding 100k elements to array with []

0.044686794281006

Adding 100k elements to array with array_push

0.072616100311279

Adding 100k elements to array with [] 10 per iteration

0.034690141677856

Adding 100k elements to array with array_push 10 per iteration

0.023932933807373