PHP 4.4.2:
Initialize 1 million objects: 1192.52 ms
Initialize 1 million arrays: 735.15ms
Fetch object: 2.34ms
Fetch array: 0.81ms
Make object with 1,000 properties: 3ms
Make array with 1,000 keys: 2.51ms
Foreach through 1,000 object properties: 3.78ms
Foreach through 1,000 array keys: 2.6ms 

PHP 5.1.2:
Initialize 1 million objects: 811.72 ms
Initialize 1 million arrays: 281.83ms 
Fetch object: 0.94ms 
Fetch array: 0.72ms 
Make object with 1,000 properties: 1.77ms 
Make array with 1,000 keys: 1.38ms 
Foreach through 1,000 object properties: 0.98ms 
Foreach through 1,000 array keys: 1.21ms