PHP benchmark: array_flip() performance dependent on array size

perf-test.php

<?php

function test($size)
{
    // prepare arrays
    $plain = [];
    for ($i = 0; $i < $size; $i++) {
        $value = rand();
        array_push($plain, $value);
    }

    // run tests
    $t1 = microtime(true);
    $x = array_flip($plain);
    $t1 = microtime(true) - $t1;

    // echo results as CSV
    printf("%s, %d, %f\n", phpversion(), $size, $t1);
}

echo "# PHP Version, Array Size, array_flip()\n";
for ($i = 1; $i <= 100000; $i = $i * 10) {
    test($i);
}

perf-test.sh

#!/bin/bash

for ver in 7.0 7.1 7.2 7.3; do
  echo "# running tests for version $ver"

  image=php:$ver-cli
  docker run --rm -v "$PWD":/src $image \
    bash -c "php /src/perf-test.php"
done

My results:

X Y Scatter Plot: