Introduction to picture and text code of PHP unlimited classification

Infinite classification means that starting from the highest classification, each sub-category can be divided into several sub-categories of its own, which can be divided all the time, which is called infinite classification;

For example, a tree A tree starts from a trunk and branches off into multiple branches, and these branches branch into other branches. Theoretically, it can be divided indefinitely;

Another example is that a family can have several children. We regard these children as the first level classification. And these children will form their own families. When they have their own offspring, it will be a second-level classification. If each family has more than one child, theoretically it can be divided indefinitely;

In php, the expression of infinite classification is as shown in the figure below;

We use the two fields of id and pid to connect the parent classification and the subset classification, so that You can find the ancestors and their descendants of a certain level of classification, for example, top classification>Chinese singer>Pop singer>Pop male singer>Jay Chou;

Just making data is not enough, we also need To display the data, at this time, we need to use the recursive algorithm of infinite classification.

Here we first explain what recursion is. Recursion is a programming technique in which a function calls itself to achieve the purpose of looping. So the question is, why don't we just use loops? If we use loops directly, we will not be able to solve the problem of parameters very well. Especially when dealing with some complex problems, additional thinking will be required when to initialize parameters and when to accumulate parameters. Not as clear as recursion.

Look at a simple recursion example;

function test(&$b=0){
    $b++;    if($b<10){
        test($b);
    }    return $b;
}echo test();

Recursion is a variant of a loop, so it also needs conditions to break out of the loop, or conditions to continue the loop. The &$b here is a way of passing parameters, which refers to the parameter itself. The parameters passed through & will continue to accumulate;

Return to the previous classification:

$host='127.0.0.1';
$db_user='root';
$db_pass='root';
$db_name='chenbk';
$timezone="Asia/Shanghai";
$link=mysqli_connect($host,$db_user,$db_pass,$db_name);
mysqli_query($link,"SET names UTF8");
header("Content-Type: text/html; charset=utf-8");

function tree($link,&$treeList=array(),$pid = 1,$count = 0) {　　//$link为数据库连接，&$treeList为输出数组，因为需要累积结果，所以加上引用
    $count+=2;　　　　//count为识别分级深度的标识
    $result = mysqli_query($link,"SELECT * FROM chen_category WHERE parentId={$pid}");
    while ($row=mysqli_fetch_assoc($result)) {
            $row['count'] = $count;
            $row['name'] = str_repeat(' ', $count).'|－'.$row['name'];　　//通过分级深度的标识，来给分类缩进效果
            $treeList[] = $row;　　　//把查询到的结果存储起来　　　　　　　　　　　　　　　　　　　　　　　　　
            tree($link,$treeList,$row['id'],$count);　　//再次调用自身，这时的pid为上一条数据的id从而找到上一条数据的子分类;
    }
    return $treeList;　　　　//输出结果
}

$values=tree($link);
echo &#39;<select name="parentId" id="">&#39;;

foreach ($values as $key => $value) {
    echo &#39;<option value="&#39;.$value[&#39;id&#39;].&#39;">&#39;.$value[&#39;name&#39;].&#39;</option>&#39;;
}

echo &#39;</select>&#39;;

This function The running process is:

1. Find the highest category through pid=0

2. Add the hierarchical depth to it, add the indentation effect through the hierarchical depth, and then save this data stand up.

3. Call yourself again, passing in the data found in 2. and the id of the first-level classification as the pid of the next-level classification; in this way, every time we find its sub-category, we will finally Find its smallest first-level classification;

The final effect is like this:

I set the default value of pid here to 1, so the highest The classification is not displayed;

As for the sorting effect of this algorithm, it lies in the wilhe loop. When calling the function itself, the first data we found was 2 pieces, one for a Chinese singer and one for a European and American singer. However, , during operation, the function is called hierarchically, just like a multi-layer for loop. It will wait until the innermost loop ends before completing the outer loops in sequence. Therefore, when we found the Chinese singer, the function did not end. The recursion itself found the pop singer. It was not until we found Jay Chou that we started to process the pop female singer on the upper layer. The sorting is completed based on the results of this process.

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