Detailed explanation of usage examples of PHP conditional statements if else and switch

In php, conditional statements mainly use statements such as if else, if elseif, and switch case. These two statements are the most used.

1. if...else statement

if...else statement

Execute a block of code when the condition is true, and execute another block of code when the condition is not true

Grammar:

1. Simple condition

if(条件){
        ……
    }
    else{
        ……
    }

<?php
if (date("D") == "Sat") echo " 提醒你周末了，狂欢去";
?>

Example: The execution part of this example has three lines, and the curly brackets cannot be omitted.

<?php
if (file_exists("/usr/local/lib/php3.ini")) {
  echo "以下是 PHP3 的配置文件<p><pre class="brush:php;toolbar:false">n";
  readfile("/usr/local/lib/php3.ini");
  echo "

n"; } ?>

2. Complex conditions

elseif statement

is used in conjunction with if...else to execute a code block when one of several conditions is true

if(条件){
        ……
    }
    elseif(条件){
        ……
    }
    else{
        ……
}

Example: Modify the above example into a more complete processing. Since there is only one line of instructions for executing else, there is no need to add braces.

<?php
$f="/usr/local/lib/php3.ini";
if (file_exists($f)) {
  echo "以下是 PHP3 的配置文件<p><pre class="brush:php;toolbar:false">n";
  readfile($f);
  echo "

n"; } else echo "很抱歉，找不到 $f"; ?>

The third type is the recursiveif..else loop, which is usually used in various decision-making judgments. It combines several if..else statements for processing.

Look directly at the example below

<?php
if ($a > $b) {
  echo "a 比 b 大";
} elseif ($a == $b) {
  echo  "a 等于 b";
} else {
  echo "a 比 b 小";
}
?>

The above example only uses a two-level if..else loop to compare the two variables a and b. When actually using this kind of recursive if..else loop, please use it with caution, because too many levels of loops can easily cause problems with the design logic, or missing braces, etc., can cause inexplicable problems in the program.

2. Switch statement

1 Syntax:

switch(表达式){
    case 值1:
                    语句
                    break;
    case 值2:
                    语句
                    break;
    default:
                没有匹配的值时执行的语句
}

Working principle:

1. For expressions (usually variables ) Perform a calculation
2. Compare the value of the expression with the value of the case in the structure
3. If there is a match, execute the code associated with the case
4. After the code is executed, the break statement blocks The code jumps to the next case and continues execution
5. If no case is true, use the default statement

Use switch to implement a multi-purpose page. First create the test.php page:

<?php
echo "<a href=&#39;solution.php?action=add&#39;>增加</a><br><br>";
echo "<a href=&#39;solution.php?action=del&#39;>删除</a><br><br>";
echo "<a href=&#39;solution.php?action=search&#39;>查找</a><br><br>";
echo "<a href=&#39;solution.php?action=update&#39;>更新</a>";
?>

Next, let's see how solution.php handles these four operations.

<?php
$action=$_GET["action"];
switch ($action)
{
 case "add":
 echo "现在可以实现增加功能！";
 break;
 case "del":
 echo "现在可以实现删除功能！";
 break;
 case "search":
 echo "现在可以实现查询功能！";
 break;
 case "update":
 echo "现在可以实现更新功能！";
 break;
}
?>

It’s very simple. We first receive the value of action, and use the switch statement to assign corresponding operations to it according to the different action values.

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