When looking for something in an array of values, it is very tempting to use in_array(). After all, that’s what the name says. However, searching through an array, even with best-case search algorithms, will never be faster than a single index lookup, which is where isset() comes in. With isset(), you can use one operation to see if a value exists, provided those values exist as keys. I don’t know if it’s truly random access, but it’s pretty darn close.

So, instead of something like this:

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$exclude = array(1, 4, 6, 8);
 
for ($i = 0, $size = count($data); $i < $size; $i++)
{
   if (in_array($data[$i]['id'], $exclude)
   {
      // do something
   }
}

do something like:

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$exclude[1] = true;
$exclude[4] = true;
$exclude[6] = true;
$exclude[8] = true;
 
for ($i, $size = count($data); $i < $size; $i++)
{
   if (isset($exclude[$data[$i]['id']]))
   {
      // do something
   }
}

So does this make a difference? Let’s write a little benchmark script.

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#!/usr/bin/php
<?php
$haystack = array();
for ($i = 0; $i < 1000; $i++)
{
    $haystack[] = rand(0, 1000);
}
 
$needles = array();
for ($i = 0; $i < 1000; $i++)
{
    $needles[] = rand(0, 1000);
}
 
for ($i = 0; $i < 1000; $i++)
{
    foreach ($needles as $needle)
    {
        if (in_array($needle, $haystack));
    }
}

We fill two arrays with 1000 random integers. One is the haystack – what we will search through. The other is the list of needles – we want to search for each one. For each needle, we look for it in the haystack. Then, we repeat this 1000 times.

Executing this, the script takes around 37 seconds:

% time ./bench.php
 
real    0m37.400s
user    0m37.282s
sys     0m0.068s

Now, let’s change the last for() loop to this:

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for ($i = 0; $i < 1000; $i++)
{
    $tmp = array_flip($haystack);
    foreach ($needles as $needle)
    {
        if (isset($tmp[$needle]));
    }
}

The new output:

% time ./bench.php
 
real    0m0.778s
user    0m0.764s
sys     0m0.008s

Execution time drops from around 37 seconds to 0.7 seconds.