PHP foreach usage and examples

Original URL:
http://blog.csdn.net/china_skag/article/details/6444727
PHP 4 introduced the foreach structure, which is very similar to 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.

Let’s look at the first statement first. This statement is relatively simple. array_expression refers to an array expression. The as $val statement will sequentially obtain the value of the array and save it to the $val variable. This method can only obtain the array. The value within the array cannot be obtained, but the subscript index value of the array cannot be obtained. 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 also The index value of the element can be obtained and saved in the $key variable. If the index value of the array has not been manually set, the system default setting value will be returned.
Look at the positive example:
Let’s first look at a simple one-dimensional array:
$myArray=array("1"=>"val1","2"="val2","3"=>"val3");
foreach($myArray as $key=>$val) {
  print($key."=>".$val.";");
}
The program will output: 1=>val1;2=>val2;3=>val3;, next Let's look at a more complicated two-dimensional array traversal. The program is as follows:
$myArray=array(
"1"=>array("11"=>"val11","12"=>"val12", "13"=>"val13"),
"2"=>array("21"=>"val21","22"=>"val22","23"=>"val23") ,
      "3"=>array("31"=>"val31","32"=>"val32","33"=>"val33")
);
print("foreach($myArray as $key=>$val) {
print("

".$key."

");
if (is_array($val)) {                                                                                              Determine whether the value of $val is an array, if so, enter the lower level traversal
                                                                                                                                         

".$key."=>".$val."

");
                                                                                                                                                                                     ;/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 a solid dot and a hollow dot.
  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.
  Another example to solve the puzzle
  $a = array("1"=>"Chinese","2"=>"Mathematics","3"=>"English");
  $b = array("1"=>"95","2"=>"99","3"=>"92");
  foreach($a as $key=>$value) {
  echo $value;
  echo $b[$key]."
  ";
  }
  ?>
  The question is why the value in the output array $b should be $b[$key] instead of $ b[$value]?
  Why is this?
  $a = array("1"=>"Chinese","2"=>"Mathematics","3"=>"English");
  The above one is exactly the same as the one below
  $a [1]="Chinese";
  $a[2]="Mathematics"
  $a[3]="English"
  How do we output the above array?
  It must be echo $a[1];
  right?
  If there is no doubt, let’s continue! ! ! !
  ------------------------------
  Simply say foreach
  The format is like this foreach(array name as subscript = >Value)
  The subscript is the $a[1] above, and the 1 here is the subscript of the array!
  By now you should understand why $a[$key] is output like this
  Please remember that no matter how it changes, the output method of the array will always be $a[1], not $a['Chinese']
  = ================================================== =============
  foreach() has two uses:
  1: foreach(array_name as $value){
          statement;
      }
  The array_name here is the name of the array you want to traverse, In each loop, the value of the current element of the array_name array is assigned to $value, and the subscript inside the array moves down one step, that is, the next element is obtained in the next loop. C2: Foreach (array_name as $ key = & gt; $ value) {
  statement;
  }
  The difference between the first method here is $ key, that is, in addition to paying the value of the current element to $ value , the key value of the current element will also be assigned to the variable $key in each loop. The key value can be a subscript value or a string. For example, "0" in book[0]=1, "id" in book[id]="001".
  
  The above introduces the usage and examples of PHP foreach, including the relevant content. I hope it will be helpful to friends who are interested in PHP tutorials.