What is the usage of foreach in php

PHP 4 introduced the foreach construct, much like Perl and other languages. This is just a convenient way to iterate over an array. foreach can only be used with arrays, and an error will occur when trying to use it with other data types or an uninitialized variable. There are two syntaxes, the second being a less important but useful extension of the first.

foreach (array_expression as $value)
    statement
foreach (array_expression as $key => $value)
    statement

The first format iterates over the given array_expression array. Each time through the loop, the value of the current cell is assigned to $value and the pointer inside the array is moved forward one step (so the next cell will be obtained in the next loop).

The second format does the same thing, except that the key name of the current unit will also be assigned to the variable $key in each loop.

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Let’s look at the first statement first. This statement is relatively simple. array_expression refers to an array expression, as $ The val statement will sequentially obtain the values ​​of the array and save them to the $val variable. This method can only obtain the values ​​in the array, but not the subscript index value of the array. For example:

$myArray=array("1"=>"val1","2"=>"val2","3"=>"val3");
foreach($myArray as $val) {
     print($val." ");
}

The result will be output: val1 val2 val3

Let’s look at the second format. In addition to getting the value of the elements in the array like the first format, the second format can , you can also get the index value of the element and save it to the $key variable. If the index value of the array has not been manually set, it will return to the system default setting value.

See the positive example:

Let’s look at a simple one-dimensional array first:

$myArray=array("1"=>"val1","2"="val2","3"=>"val3");
foreach($myArray as $key=>$val) {
     print($key."=>".$val.";");
}

The program will output: 1=>val1;2=>val2;3=>val3;, let’s look at another one A more complex two-dimensional array traversal, the program is as follows:

$myArray=array(
     "1"=>array("11"=>"val11","12"=>"val12","13"=>"val13"),
     "2"=>array("21"=>"val21","22"=>"val22","23"=>"val23"),
     "3"=>array("31"=>"val31","32"=>"val32","33"=>"val33")
);
print("<ul>");
foreach($myArray as $key=>$val) {
     print("<li>".$key."</li>");
     if (is_array($val)) {     //判断$val的值是否是一个数组，如果是，则进入下层遍历
         print("<ul>");
        foreach($val as $key=>$val) {
             print("<li>".$key."=>".$val."</li>");
         }
         print("</ul>");
     }
}
print("</ul>");

Output result:

·1
    ·11=>val11
    ·12=>val12
    ·13=>val13
·2
    ·21=>val21
    ·22=>val22
    ·23=>val23
·3
    ·31=>val31
    ·32=>val32
    ·33=>val33

and
• are labels, which are used to display solid dots and hollow dots Small dots.

  Since the above is a two-dimensional array, the $val value obtained after the first traversal will be an array, so I added a judgment to the traversal for second-level array traversal.

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