Is micro-optimization worth the time?
As the cliche goes, micro-optimization is generally worth the time only in the smallest, most performance-critical hotspots of your code, only after you've proven that's where the bottleneck is. However, I'd like to flesh this out a little, to point out some exceptions and areas of misunderstanding.
This doesn't mean that performance should not be considered at all upfront. I define micro-optimization as optimizations based on low-level details of the compiler/interpreter, the hardware, etc. By definition, a micro-optimization does not affect big-O complexity. Macro-optimizations should be considered upfront, especially when they have a major impact on high-level design. For example, it's pretty safe to say that if you have a large, frequently accessed data structure, an O(N) linear search isn't going to cut it. Even things that are only constant terms but have a large and obvious overhead might be worth considering upfront. Two big examples are excessive memory allocation/data copying and computing the same thing twice when you could be computing it once and storing/reusing the result.
If you're doing something that's been done before in a slightly different context, there may be some bottlenecks that are so well-known that it's reasonable to consider them upfront. For example, I was recently working on an implementation of the FFT (fast Fourier Transform) algorithm for the D standard library. Since so many FFTs have been written in other languages before, it's very well-known that the biggest bottleneck is cache performance, so I went into the project immediately thinking about how to optimize this.
Micro-optimisation is worth it when you have evidence that you're optimising a bottleneck.
Usually it's not worth it - write the most readable code you can, and use realistic benchmarks to check the performance. If and when you find you've got a bottleneck, micro-optimise just that bit of code (measuring as you go). Sometimes a small amount of micro-optimisation can make a huge difference.
But don't micro-optimise all your code... it will end up being far harder to maintain, and you'll quite possibly find you've either missed the real bottleneck, or that your micro-optimisations are harming performance instead of helping.
Is micro-optimization worth the time?
No, unless it is.
In other words, a-priori, the answer is "no", but after you know a specific line of code consumes a healthy percent of clock time, then and only then is it worth optimizing.
In other words, profile first, because otherwise you don't have that knowledge. This is the method I rely on, regardless of language or OS.
Added: When many programmers discuss performance, from experts on down, they tend to talk about "where" the program spends its time. There is a sneaky ambiguity in that "where" that leads them away from the things that could save the most time, namely, function call sites. After all, the "call Main" at the top of an app is a "place" that the program is almost never "at", but is responsible for 100% of the time. Now you're not going to get rid of "call Main", but there are nearly always other calls that you can get rid of. While the program is opening or closing a file, or formatting some data into a line of text, or waiting for a socket connection, or "new"-ing a chunk of memory, or passing a notification throughout a large data structure, it is spending great amounts of time in calls to functions, but is that "where" it is? Anyway, those calls are quickly found with stack samples.
Well, for a trivially small array, $array === (array) $array is significantly faster than is_array($array). On the order of over 7 times faster. But each call is only on the order of 1.0 x 10 ^ -6 seconds (0.000001 seconds). So unless you're calling it literally thousands of times, it's not going to be worth it. And if you are calling it thousands of times, I'd suggest you're doing something wrong...
The difference comes when you're dealing with a large array. Since $array === (array) $array requires a new variable to be copied requires the array to be iterated over internally for the comparison, it'll likely be SIGNIFICANTLY slower for a large array. For example, on an array with 100 integer elements, is_array($array) is within a margin of error (< 2%) of is_array() with a small array (coming in at 0.0909 seconds for 10,000 iterations). But $array = (array) $array is extremely slow. For only 100 elements, it's already over twice as slow as is_array() (coming in at 0.203 seconds). For 1000 elements, is_array stayed the same, yet the cast comparison increased to 2.0699 seconds...
The reason it's faster for small arrays is that is_array() has the overhead of being a function call, where the cast operation is a simple language construct... And iterating over a small variable (in C code) will typically be cheaper than the function call overhead. But, for larger variables, the difference grows...
It's a tradeoff. If the array is small enough, iteration will be more efficient. But as the size of the array grows, it will becomes increasingly slower (and hence the function call will become faster).
Another Way To Look At It
Another way to look at it would be to examine the algorithmic complexity of each cast.
Let's take a look at is_array() first. It's source code basically shows it's an O(1) operation. Meaning it's a constant time operation. But we also need to look at the function call. In PHP, function calls with a single array parameter are either O(1) or O(n) depending on if copy-on-write needs to be triggered. If you call is_array($array) when $array is a variable reference, copy-on-write will be triggered and a full copy of the variable will occur.
So therefore is_array() is a best case O(1) and worst-case O(n). But as long as you're not using references, it's always O(1)...
The cast version, on the other hand, does two operations. It does a cast, then it does an equality check. So let's look at each separately. The cast operator handler first forces a copy of the input variable. No matter if it's a reference or not. So simply using the (array) casting operator forces an O(n) iteration over the array to cast it (via the copy_ctor call).
Then, it converts the new copy to an array. This is O(1) for arrays and primitives, but O(n) for objects.
Then, the identical operator executes. The handler is just a proxy to the is_identical_function(). Now, is_identical will short-circuit if $array is not an array. Therefore, it has a best case of O(1). But if $array is an array, it can short-circuit again if the hash tables are identical (meaning both variables are copy-on-write copies of each other). So that case is O(1) as well. But remember that we forced a copy above, so we can't do that if it's an array. So it's O(n) thanks to zend_hash_compare...
So the end result is this table of worst-case runtime:
+----------+-------+-----------+-----------+---------------+
| | array | array+ref | non-array | non-array+ref |
+----------+-------+-----------+-----------+---------------+
| is_array | O(1) | O(n) | O(1) | O(n) |
+----------+-------+-----------+-----------+---------------+
| (array) | O(n) | O(n) | O(n) | O(n) |
+----------+-------+-----------+-----------+---------------+
Note that it looks like they scale the same for references. They don't. They both scale linearly for referenced variables. But the constant factor changes. For example, in a referenced array of size 5, is_array will perform 5 memory allocations, and 5 memory copies, followed by 1 type check. The cast version, on the other hand, will perform 5 memory allocations, 5 memory copies, followed by 2 type checks, followed by 5 type checks and 5 equality checks (memcmp() or the like). So n=5 yields 11 ops for is_array, yet 22 ops for ===(array)...
Now, is_array() does have the O(1) overhead of a stack push (due to the function call), but that'll only dominate runtime for extremely small values of n (we saw in the benchmark above just 10 array elements was enough to completely eliminate all difference).
The Bottom Line
I'd suggest going for readability though. I find is_array($array) to be far more readable than $array === (array) $array. So you get the best of both worlds.
The script I used for the benchmark:
$elements = 1000;
$iterations = 10000;
$array = array();
for ($i = 0; $i < $elements; $i++) $array[] = $i;
$s = microtime(true);
for ($i = 0; $i < $iterations; $i++) is_array($array);
$e = microtime(true);
echo "is_array completed in " . ($e - $s) ." Seconds\n";
$s = microtime(true);
for ($i = 0; $i < $iterations; $i++) $array === (array) $array;
$e = microtime(true);
echo "Cast completed in " . ($e - $s) ." Seconds\n";
Edit: For the record, these results were with 5.3.2 on Linux...
Edit2: Fixed the reason the array is slower (it's due to the iterated comparison instead of memory reasons). See compare_function for the iteration code...