Detailed explanation of PHP citation - pitfalls and magical uses

Recently, I have been paying attention to the topic of "PHP Reference". I have read many in-depth articles and have a deeper understanding of "Reference" in PHP.

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First look at the following code:

$foo['hello'] = '0'; $bar = &$foo['hello']; // 引用！ $tipi = $foo; $tipi['hello'] = '1'; print_r($foo);

Q: Output 0 or output 1? The answer is 1.

What’s the principle?

The PHP kernel uses thezvalstructure to store variables. In the PHP code, we use thexdebug_debug_zvalfunction to find out.

Modify the above code:

$foo['hello'] = '0'; xdebug_debug_zval('foo'); $bar = &$foo['hello']; // 引用！ xdebug_debug_zval('foo'); $tipi = $foo; $tipi['hello'] = '1'; print_r($foo);

The output is as follows:

foo: (refcount=1, is_ref=0)=array ('hello' => (refcount=1, is_ref=0)='0') foo: (refcount=1, is_ref=0)=array ('hello' => (refcount=2, is_ref=1)='0')

$foo['hello']From the non-reference variable (is_ref= 0) becomes a reference variable (is_ref=1), and the reference count isrefcount=2.

Why is this?

According toPHP: What does a quote do - Manual's explanation:

$a =& $b;This means $a and $b point to the same variable.

$a and $b are exactly the same here. It’s not that $a points to $b or vice versa, but that $a and $b point to the same place.

Combined with our example, that is, when$bar = &$foo['hello'];is executed,$barand$foo['hello']have all become "reference variables", and they "point to the same place".

Then when we copy this array, we also copy the reference of itshelloelement; when$tipi['hello'] = '1';is executed , the "same place" pointed to bytipi['hello'],$foo['hello']and$baris modified.

So, the value of$foo['hello']naturally becomes1.

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PHPers who have a little in-depth reference should have tried this syntax:

for ($list as &$value) { $value = 'foo'; }

PHP will not recycle variables after the control structure, so I won’t explain it here; So the pit just now can actually be extended.

$foo['hello'] = '0'; $foo['world'] = 'A'; foreach($foo as &$value) { // 引用！ // Do nothing. } $tipi = $foo; $tipi['hello'] = '1'; $tipi['world'] = 'B'; print_r($foo);

The output here is as follows:

Array ( [hello] => 0 [world] => B )

hellois normal, whileworldis modified toB! The reason can be explored by yourself with thexdebug_debug_zvalfunction.

So, casuallyunset($value);is a good habit.

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In fact, citations are not all pitfalls. There are still great benefits.

Example:

$catList = [ '1' => ['id' => 1, 'name' => '颜色', 'parent_id' => 0], '2' => ['id' => 2, 'name' => '规格', 'parent_id' => 0], '3' => ['id' => 3, 'name' => '白色', 'parent_id' => 1], '4' => ['id' => 4, 'name' => '黑色', 'parent_id' => 1], '5' => ['id' => 5, 'name' => '大', 'parent_id' => 2], '6' => ['id' => 6, 'name' => '小', 'parent_id' => 2], '7' => ['id' => 7, 'name' => '黄色', 'parent_id' => 1], ];

How to convert the above sequence table into a hierarchical tree?

In the past, or usually the first thing we thought of was recursive backtracking.

However, using the reference feature of PHP, the time complexity can be reduced toO(n).

$treeData = []; foreach ($catList as $item) { if (isset($catList[$item['parent_id']]) && !empty($catList[$item['parent_id']])) { // 子分类 $catList[$item['parent_id']]['children'][] = &$catList[$item['id']]; } else { // 一级分类 $treeData[] = &$catList[$item['id']]; } } var_export($treeData);

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