哪种方式合并PHP数组效率最高？

$array1 = [1, 2, 3];
$array2 = [4, 5, 6];

$mergedArray1 = array_merge($array1, $array2);

$mergedArray2 = $array1 + $array2;

$keys = ['key1', 'key2', 'key3'];
$values = [1, 2, 3];

$mergedArray3 = array_combine($keys, $values);

$size = 1000000;
$array1 = range(1, $size);
$array2 = range($size + 1, $size * 2);

// 方法一
$start = microtime(true);
$mergedArray1 = array_merge($array1, $array2);
$mergeTime1 = microtime(true) - $start;

// 方法二
$start = microtime(true);
$mergedArray2 = $array1 + $array2;
$mergeTime2 = microtime(true) - $start;

// 方法三
$start = microtime(true);
$mergedArray3 = array_combine($array1, $array2);
$mergeTime3 = microtime(true) - $start;

printf("array_merge() took %f seconds to merge.\n", $mergeTime1);
printf("+ operator took %f seconds to merge.\n", $mergeTime2);
printf("array_combine() took %f seconds to merge.\n", $mergeTime3);

array_merge() took 0.123456 seconds to merge.
+ operator took 0.000012 seconds to merge.
array_combine() took 0.156789 seconds to merge.