对于foreach（在这种情况下是php5）的数组的'副本'是具有实际开销的立即副本，还是仅仅是一个惰性副本（写入时复制），如果它检测到写入操作只会产生开销？
备选方案，在几个地方注意，是运行foreach over keys（$ array） - 这怎么可能真的更快？
答案 0 :(得分：2)
好的，所以我去了测量它 -
Test This And This And, uh, This
--------------------------- ---------------------------- ----------------------------
Setup Setup Setup
---------------------------- ---------------------------- ----------------------------
$i = array_fill(0,1000,'1'); $j = array_fill(0,1000,'1'); $j = array_fill(0,1000,'1');
$c = 0; $d = 0; $e = 0;
--------------------------- ---------------------------- ---------------------------
Code Under Test Code Under Test Code Under Test
--------------------------- ------------------------------ --------------------------
foreach ($i as $v) foreach (array_keys($j) as $k) foreach ($i as &$v)
{ $c+= $v; } { $d+= $j[$k]; } { $e+= $v; }
--------------------------- ------------------------------ -------------------------
Tear Down Tear Down Tear Down
--------------------------- ----------------------------- -------------------------
print "c = $c"; print "d = $d"; print "e = $e";
---------------------------- ----------------------------- --------------------------
Test for repetitions. 10000
---------------------------- ----------------------------- -------------------------
c = 10000000 d = 10000000 e = 10000000
---------------------------- ----------------------------- -------------------------
Ran in 1.8540189266205 Ran in 4.0039160251617 Ran in 1.9633851051331
--------------------------- ----------------------------- -------------------------
Winner -0.10936617851257 Looser 2.0405309200287 2nd Best 0
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看起来foreach（$ a as $ v）比array_keys好得多，使用＆amp; v就在中间。
答案 1 :(得分：1)
PHP使用copy-on-write。通过引用传递的速度较慢，因为它必须设置数据结构以维护引用。