为了评估传引用的性能和传值的性能，我使用了此脚本。以下是结论。
#!/usr/bin/php
<?php
function sum($array,$max){ //For Reference, use: "&$array"
$sum=0;
for ($i=0; $i<2; $i++){
#$array[$i]++; //Uncomment this line to modify the array within the function.
$sum += $array[$i];
}
return ($sum);
}
$max = 1E7 //10 M data points.
$data = range(0,$max,1);
$start = microtime(true);
for ($x = 0 ; $x < 100; $x++){
$sum = sum($data, $max);
}
$end = microtime(true);
echo "Time: ".($end - $start)." s\n";
/* Run times:
# PASS BY MODIFIED? Time
- ------- --------- ----
1 value no 56 us
2 reference no 58 us
3 valuue yes 129 s
4 reference yes 66 us
Conclusions:
1. PHP is already smart about zero-copy / copy-on-write. A function call does NOT copy the data unless it needs to; the data is
only copied on write. That's why #1 and #2 take similar times, whereas #3 takes 2 million times longer than #4.
[You never need to use &$array to ask the compiler to do a zero-copy optimisation; it can work that out for itself.]
2. You do use &$array to tell the compiler "it is OK for the function to over-write my argument in place, I don't need the original
any more." This can make a huge difference to performance when we have large amounts of memory to copy.
(This is the only way it is done in C, arrays are always passed as pointers)
3. The other use of & is as a way to specify where data should be *returned*. (e.g. as used by exec() ).
(This is a C-like way of passing pointers for outputs, whereas PHP functions normally return complex types, or multiple answers
in an array)
4. It's unhelpful that only the function definition has &. The caller should have it, at least as syntactic sugar. Otherwise
it leads to unreadable code: because the person reading the function call doesn't expect it to pass by reference. At the moment,
it's necessary to write a by-reference function call with a comment, thus:
$sum = sum($data,$max); //warning, $data passed by reference, and may be modified.
5. Sometimes, pass by reference could be at the choice of the caller, NOT the function definitition. PHP doesn't allow it, but it
would be meaningful for the caller to decide to pass data in as a reference. i.e. "I'm done with the variable, it's OK to stomp
on it in memory".
*/
?>