I benchmarked performance of arrays vs stdClass instances. I started with stdClass, and I knew it was going to be bad. The code was run like so:

$ time php blah.php

And the times were generated by running this three times and averaging the duration.

For arrays: .778 seconds

For objects: 6.516 seconds.

Ouch! Eight times slower. Here’s the code:

<?php

for ($i = 0; $i < 1000000; $i++) {
    $x = new stdClass;
    $x->a = 'a';
    $x->b = 'b';
}

?>

<?php

for ($i = 0; $i < 1000000; $i++) {
    $x = array();
    $x['a'] = 'a';
    $x['b'] = 'b';
}

?>

The other thing is that there’s no object literal syntax in PHP like there is for arrays, so they can be less convenient. The array code is also slightly faster when using literal syntax: $x = array('a' => 'a', 'b' => 'b').