is quicker to use array_keys() in a foreach statemnt or set a value variable that's never used?

Question

Suppose I want to iterate over an array and either I never look at the values, or I am setting things in them, so I only want the keys. Which is quicker:

// Set a variable each iteration which is unused.
foreach ($array as $key => $value) {
  $array[$key]['foo'] = 'bar';
}

// Call array_keys() before iterating.
foreach (array_keys($array) as $key) {
  $array[$key]['foo'] = 'bar';
}

Answer 1

I think this would also work, and may be quicker:

foreach ($array as &$value) {
  $value['foo'] = 'bar';
}

UPDATE: I did a little test, and it seems this is faster. http://codepad.org/WI7Mtp8K

Answer 2

Leaving aside that the second example is contrived. Meaning, as Ashley Banks noted, why would you be doing this?

The first example would be more performant of the two. The second has the overhead of the additional function call to array_keys().

Check out the PHP Benchmark for additional performance tests. When in doubt, benchmark it yourself with microtime().