Question

What is quickest? To do a condition based on a variable outside the loop inside a loop or outside, or does it even matter (does the compiler do it for you), or should you maybe use a completely different workaround?

Example #1 with the condition inside the loop (a single foreach):

$test = 2;  
foreach ($list as $listItem) {
    if ($test == 1) {
        $listItem .= " - one";
    } else if ($test == 2) {
        $listItem .= " - two";
    } else if ($test == 3) {
        $listItem .= " - three";
    }
}

Example #2 with the condition outside the loop (quite ugly with multiple foreach’s):

$test = 2;  
if ($test == 1) {
    foreach ($list as $listItem) {
        $listItem .= " - one";
    }
} else if ($test == 2) {
    foreach ($list as $listItem) {
        $listItem .= " - two";
    }
} else if ($test == 3) {
    foreach ($list as $listItem) {
        $listItem .= " - three";
    }
}

Answer 1

As you probably could’ve guessed, example #2 is quicker, and the compiler doesn’t do it for you.

Overview

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Performance tests

Example #1, condition inside of loop:

Time taken: 0.2501 seconds
Time taken: 0.2336 seconds
Time taken: 0.2335 seconds
Time taken: 0.2319 seconds
Time taken: 0.2337 seconds 
Average:    0.2364 seconds

And the code:

<?php
    $list = array();
    for ($i = 0; $i < 1000000; $i++) {
        $list[] = md5(rand(0, 100000) * $i);
    }
    $a = microtime(true);
    $test = 2;
    foreach ($list as $listItem) {
        if ($test == 1) {
            $listItem .= " - one";
        } else if ($test == 2) {
            $listItem .= " - two";
        } else if ($test == 3) {
            $listItem .= " - three";
        }
    }
    echo "Time taken: " . number_format(microtime(true) - $a, 4) . " seconds";
?>

Example #2, condition outside of loop:

Time taken: 0.1424 seconds
Time taken: 0.1426 seconds
Time taken: 0.1364 seconds
Time taken: 0.1348 seconds
Time taken: 0.1347 seconds
Average:    0.1382 seconds

And the code:

<?php
    $list = array();
    for ($i = 0; $i < 1000000; $i++) {
        $list[] = md5(rand(0, 100000) * $i);
    }
    $a = microtime(true);
    $test = 2;
    if ($test == 1) {
        foreach ($list as $listItem) {
            $listItem .= " - one";
        }
    } else if ($test == 2) {
        foreach ($list as $listItem) {
            $listItem .= " - two";
        }
    } else if ($test == 3) {
        foreach ($list as $listItem) {
            $listItem .= " - three";
        }
    }
    echo "Time taken: " . number_format(microtime(true) - $a, 4) . " seconds";
?>

Alright, that’s a pretty obvious win for conditions outside of loops, but what if we try to compare with a boolean instead of an int?

Example #3, condition inside of loop, but conditioning with a boolean instead:

Time taken: 0.1845 seconds
Time taken: 0.1821 seconds
Time taken: 0.1745 seconds
Time taken: 0.1777 seconds
Time taken: 0.1767 seconds
Average:    0.1791 seconds

And the code:

<?php
    $list = array();
    for ($i = 0; $i < 1000000; $i++) {
        $list[] = md5(rand(0, 100000) * $i);
    }
    $a = microtime(true);
    $test = 2;
    $result1 = ($test == 1);
    $result2 = ($test == 2);
    $result3 = ($test == 3);
    foreach ($list as $listItem) {
        if ($result1) {
            $listItem .= " - one";
        } else if ($result2) {
            $listItem .= " - two";
        } else if ($result3) {
            $listItem .= " - three";
        }
    }
    echo "Time taken: " . number_format(microtime(true) - $a, 4) . " seconds";
?>

Interesting. What if we use a built in function such as array_walk?

Example #4, array_walk:

Time taken: 0.4950 seconds
Time taken: 0.4946 seconds
Time taken: 0.4947 seconds
Time taken: 0.4937 seconds
Time taken: 0.4918 seconds
Average:    0.4940 seconds

And the code:

<?php
    function append_string($value, $suffix) {
        return $value . $suffix;
    }

    $list = array();
    for ($i = 0; $i < 1000000; $i++) {
        $list[] = md5(rand(0, 100000) * $i);
    }
    $a = microtime(true);
    $test = 2;
    if ($test == 1) {
        array_walk($list, "append_string", " - one");
    } else if ($test == 2) {
        array_walk($list, "append_string", " - two");
    } else if ($test == 3) {
        array_walk($list, "append_string", " - three");
    }
    echo "Time taken: " . number_format(microtime(true) - $a, 4) . " seconds";
?>

What?! You might think an inbuilt function would abstract it over a C or C++ function, and it might very well, however the problem is that the function calls make this method very very slow.

---

Pros/cons

Example #1 pros

• You don’t have to declare the loop multiple times (which might be annoying if you have several conditions)

Example #1 cons

• It is the slowest one of all the first 3 tests (71% slower than example #2 and 32% slower than example #3)

Example #2 pros

• The very quickest one of them all

Example #2 cons

• It requires you to declare the loop several times (as many times as the amount of conditions you have)

Example #3 pros

• You don’t have to declare the loop multiple times, just like in example #1
• It is quicker than #example 1

Example #3 cons

• It is still slower than #example 2 (about 30%)
• It looks more messy

Example #4 pros

• The syntax looks pretty and easy

Example #4 cons

• It is the slowest one of the lot (about 100% slower than the second slowest (example #1)). Possibly due to it having to call a function 1,000,000 times.
• You’re out of the global scope and will have to global all variables from the global scope.

---

Conclusion

The PHP compiler doesn’t do it for you, and if you want performance you should go for example #2, however this test is made with an array that has a million entries. Chances are that your array isn’t going to have that many entries, and therefore you might go with example #1 or example #3 instead. Oh, and do not go for example #4.