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Array intersection benchmarks

I’m trying to optimize my hot code path where array intersection is used a lot. I got curious and decided to compare the various intersection algorithms that I know of.

<?php

// Source - https://stackoverflow.com/a/9276284
// Posted by kingmaple, modified by community. See post 'Timeline' for change history
// Retrieved 2026-04-08, License - CC BY-SA 4.0

// Source - https://stackoverflow.com/a/53203232
// Posted by slaszu, modified by community. See post 'Timeline' for change history
// Retrieved 2026-04-08, License - CC BY-SA 4.0

ini_set('memory_limit', '2048M');

function formatBytes(int $bytes): string {
$units = ['B', 'KB', 'MB', 'GB'];
$i = 0;
while ($bytes >= 1024 && $i < count($units) - 1) {
$bytes /= 1024;
$i++;
}
return sprintf("%.2f %s", $bytes, $units[$i]);
}

function benchmark(callable $fn, string $label): array {
gc_collect_cycles();
gc_mem_caches();
memory_reset_peak_usage();
$mem = -memory_get_peak_usage();
$time = -hrtime(true);
$fn();
$time += hrtime(true);
$mem += memory_get_peak_usage();
return [
'label' => $label,
'time_ms' => $time / 1e6,
'mem_used' => $mem,
];
}

function manual_intersect($arrayOne, $arrayTwo) {
$index = array_flip($arrayOne);
foreach ($arrayTwo as $value) {
if (isset($index[$value])) {
unset($index[$value]);
}
}
foreach ($index as $value => $key) {
unset($arrayOne[$key]);
}
return $arrayOne;
}

function flipped_intersect($arrayOne, $arrayTwo) {
$index = array_flip($arrayOne);
$second = array_flip($arrayTwo);
$x = array_intersect_key($index, $second);
return array_flip($x);
}

function runBenchmarks(int $n): void {
echo "\n=== Array Intersection Benchmark for " . number_format($n) . " elements ===\n";

// Generate test arrays
$one = [];
$two = [];
for ($i = 0; $i < $n; $i++) {
$one[] = rand(0, 1000000);
$two[] = rand(0, 100000);
$two[] = rand(0, 10000);
}

$one = array_unique($one);
$two = array_unique($two);

$results = [];

$results[] = benchmark(
fn() => $res = manual_intersect($one, $two),
'manual_intersect()'
);

$results[] = benchmark(
fn() => $res = array_intersect($one, $two),
'array_intersect()'
);

$results[] = benchmark(
fn() => $res = flipped_intersect($one, $two),
'flipped_intersect()'
);

// --- Print Table ---
echo str_repeat('-', 60) . "\n";
printf("%-25s | %-14s | %-15s\n", 'Method', 'Time (ms)', 'Memory');
echo str_repeat('-', 60) . "\n";

foreach ($results as $r) {
printf("%-25s | %11.3f ms | %15s\n",
$r['label'],
$r['time_ms'],
formatBytes($r['mem_used'])
);
}
echo str_repeat('-', 60) . "\n";
}

// Run for various sizes
foreach ([20, 20000, 200000, 1000000] as $n) {
runBenchmarks($n);
}

I run this on PHP 8.4 on Core I7 11700F

=== Array Intersection Benchmark for 20 elements ===
------------------------------------------------------------
Method | Time (ms) | Memory
------------------------------------------------------------
manual_intersect() | 0.007 ms | 1.98 KB
array_intersect() | 0.029 ms | 3.02 KB
flipped_intersect() | 0.002 ms | 3.97 KB
------------------------------------------------------------

=== Array Intersection Benchmark for 20,000 elements ===
------------------------------------------------------------
Method | Time (ms) | Memory
------------------------------------------------------------
manual_intersect() | 1.169 ms | 1.75 MB
array_intersect() | 41.300 ms | 1.88 MB

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