Why is casting and comparing in PHP faster than is_*?

Question

While optimizing a function in PHP, I changed

if(is_array($obj)) foreach($obj as $key=>$value { [snip] } 
else if(is_object($obj)) foreach($obj as $key=>$value { [snip] } 

to

if($obj == (array) $obj) foreach($obj as $key=>$value { [snip] } 
else if($obj == (obj) $obj) foreach($obj as $key=>$value { [snip] } 

After learning about ===, I changed that to

if($obj === (array) $obj) foreach($obj as $key=>$value { [snip] } 
else if($obj === (obj) $obj) foreach($obj as $key=>$value { [snip] } 

Changing each test from is_* to casting resulted in a major speedup (>30%).

I understand that === is faster than == as no coercion has to be done, but why is casting the variable so much faster than calling any of the is_*-functions?

Edit: Since everyone asked about correctness, I wrote this little test:

$foo=(object) array('bar'=>'foo');
$bar=array('bar'=>'foo');

if($foo===(array) $foo) echo '$foo is an array?';
if($bar===(object) $bar) echo '$bar is an object?';

It doesn't print any error and both variables don't get changed, so I think it's working, but I'm ready to be convinced otherwise.

Another Edit: Artefacto's program gives me the following numbers:

PHP 5.3.2-1ubuntu4.2 (64bit) on a Core i5-750 with Xdebug
Elapsed (1): 0.46174287796021 / 0.28902506828308
Elapsed (2): 0.52625703811646 / 0.3072669506073
Elapsed (3): 0.57169318199158 / 0.12708187103271
Elapsed (4): 0.51496887207031 / 0.30524897575378
Speculation: Casting and comparing can be about 1.7-4 times faster.

PHP 5.3.2-1ubuntu4.2 (64bit) on a Core i5-750 without Xdebug
Elapsed (1): 0.15818405151367 / 0.214271068573
Elapsed (2): 0.1531388759613 / 0.25853085517883
Elapsed (3): 0.16164898872375 / 0.074632883071899
Elapsed (4): 0.14408397674561 / 0.25812387466431
Without Xdebug, the extra function call didn't matter anymore, so every test (except 3) ran faster.

PHP 5.3.2-1ubuntu4.2 on a Pentium M 1.6GHz
Elapsed (1): 0.97393798828125 / 0.9062979221344
Elapsed (2): 0.39448714256287 / 0.86932587623596
Elapsed (3): 0.44513893127441 / 0.23662400245667
Elapsed (4): 0.38685202598572 / 0.82854390144348
Speculation: Casting an array is slower, casting an object can be faster, but might not be slower.

PHP 5.2.6-1+lenny8 on a Xeon 5110
Elapsed (1): 0.273758888245 / 0.530702114105
Elapsed (2): 0.276469945908 / 0.605964899063
Elapsed (3): 0.332523107529 / 0.137730836868
Elapsed (4): 0.267735004425 / 0.556323766708
Speculation: These results are similar to Artefacto's results, I think it's PHP 5.2.

The solution: The profiler I used (Xdebug) made function calls about 3 times slower (even when not profiling), but didn't affect casting and comparing noticeably, so casting and comparing appeared to be faster, even though it just wasn't affected by the debugger/profiler.

Answer 1

I can't really reproduce. In fact, your strategy gives me longer times in all but one case:

<?php

class A {
    private $a = 4;
    private $b = 4;
    private $f = 7;
}

$arr = array("a" => 4, "b" => 4, "f" => 7);

$obj = new A();

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    is_array($obj) and die("err");
}

echo "Elapsed (1.1): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    ($obj === (array) $obj) and die("err");
}

echo "Elapsed (1.2): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    is_object($arr) and die("err");
}

echo "Elapsed (2.1): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    ($arr === (object) $arr) and die("err");
}

echo "Elapsed (2.2): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    is_object($obj) or die("err");
}

echo "Elapsed (3.1): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    ($obj === (object) $obj) or die("err");
}

echo "Elapsed (3.2): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    is_array($arr) or die("err");
}

echo "Elapsed (4.1): " . (microtime(true) - $t);
echo "\n";

$t = microtime(true);

for ($i = 0; $i < 500000; $i++) {
    ($arr === (array) $arr) or die("err");
}

echo "Elapsed (4.2): " . (microtime(true) - $t);

Output:

Elapsed (1.1): 0.366055965424
Elapsed (1.2): 0.550662994385
Elapsed (2.1): 0.337422132492
Elapsed (2.2): 0.579686880112
Elapsed (3.1): 0.402997970581
Elapsed (3.2): 0.190818071365
Elapsed (4.1): 0.332742214203
Elapsed (4.2): 0.549873113632

The cast and compare was only faster for checking if something is an object. Speculation follows: perhaps because object identity checking requires only determining whether the handler tables and the object handles are the same, while checking for array identity requires, in the worst case, comparing all the values.