PHP performance issue: pass referenced variable as non-ref parameter - php

Question

function M1($x, $y){}
function M2(&$x, $y){}
function M3(&$x, &$y){}
$arr = ['a' => ['b' => range(1, 1000)]];
$ref_arr = &$arr['a'];
$var = $ref_arr['b'];
$ref = &$ref_arr['b'];
//N->N, N->N
//0.003000020980835 sec.
for($i = 0; $i < 10000; ++$i)
M1($var, $var);
//N->R, N->N (slow)
//0.59903407096863 sec.
for($i = 0; $i < 10000; ++$i)
M2($var, $var);
//N->R, N->R
//0.003000020980835 sec.
for($i = 0; $i < 10000; ++$i)
M3($var, $var);
//R->N, R->N (very slow)
//1.1980690956116 sec.
for($i = 0; $i < 10000; ++$i)
M1($ref, $ref);
//R->R, R->N (slow)
//0.58603405952454 sec.
for($i = 0; $i < 10000; ++$i)
M2($ref, $ref);
//R->R, R->R
//0.003000020980835 sec.
for($i = 0; $i < 10000; ++$i)
M3($ref, $ref);
As I know, when pass a reference variable as a non-reference parameter, PHP will copy the value. So this line takes the longest time.
M1($ref, $ref); //1.1980690956116 sec.
But how to explain the performance of these 2 lines?
M2($var, $var); //0.59903407096863 sec.
M3($var, $var); //0.003000020980835 sec.

Answer 1

After serveral years since no one give me an answer, so I decide to answer by myself.
The reason is still the COPY-ON-WRITE mechanism in PHP, but when pass a parameter with different reference type, the copy happens immediately before the function call starts.
Define:
N for non-ref variable
R for ref variable
-> for parameter passing
These 3 forms won't trigger parameter copy:
N -> N
R -> R
N (ref count = 1) -> R
And for these 2 forms a parameter copy is always happens.
R -> N
N (ref count > 1) -> R
So when you decide to call a function with reference parameter (usually array functions like sort, reset), pass an array with refcount more than 1 is not a good idea.
Bad example:
$arr1 = [];
$arr2 = $arr1;
sort($arr1);
Good example:
$arr1 = [];
$arr2 = $arr1;
unset($arr2);
sort($arr1);