What is php flow control? Understand the steps to run flow control statements

What is Flow control: In the declarative programming language, flow control instructions refer to instructions that change the order in which the program runs. It may be to run instructions at different locations, or to select one of two (or multiple) sections to run.

Imperative programming: Command the "machine" how to do things (how), so that no matter what you want (what), it will be implemented according to your command.
Declarative programming: Tell the "machine" what you want (what), and let the machine figure out how to do it (how).

Whether it is PHP or other syntax, a program always consists of several statements.

From the perspective of execution mode, the control structure of statements is divided into the following three types:

1. Sequential structure: Completely sequential execution from the first statement to the last statement;

2. Selection structure: perform several tasks based on user input or the intermediate results of statements;

3. Loop structure: based on a certain condition Simply perform a task repeatedly a number of times, or until the goal is achieved.

There are three control statements in PHP used to implement selection structures and loop structures:

1. Conditional control statements : if, else, elseif and switch;

2. Loop control statements: foreach, while, do...while..and for;

3. Transfer control statements: break, continue and return.

Here are a few small examples for reference

Conditional control statements:

If statement, usage:

　If（E）
　语句块1；
　else
　 语句块2；

Analysis: If the return value of E is true, execute statement block 1; otherwise, execute statement block 2.

Example, code:

<?php
　$a = 59;　　//根据$a的值，判断是否要妹子。如果>=60则输出要代码
　 if($a>=60){
　echo “要妹子”;
}else
　　echo “要代码”;
?>

If···elseif···else statement, usage:

if(E)
　 echo '要妹子';；
else if(X)
　 echo '不要妹子';；
else
　echo '要代码';

Analysis : If E is true, the execution is to ask for a girl. Otherwise, if the value of B is true, then no girl is required; otherwise, the executor requires code. Of course: if statements can also be nested.

The following is an example of If···elseif···else:

　<?php
　　$a = 59;
　　if($a>=60) //在大于等于60的情况里在进行分类
　{
　　if($a==100)
　　echo “要妹子”;
　　elseif($a>=90)
　　echo “睡妹子”;
　else
　　echo “睡不起”;
　}
　else
　　echo “睡大街吧”;
　?>

Switch statement, the syntax is as follows:

switch(E)
{
　case val1:
　　语句块1;
　　Break;
　case val2:
　　语句块2;
　　Break;
　default:
　　语句块3;
　}

When the value in a case statement matches the value of switch expression E, PHP starts executing the statement until the switch program section ends or the first break statement is encountered

(If break is not encountered, PHP will continue to the next case).

break is to end the entire loop body, continue is to end the word loop

The following is an example without break:

　<?php
　　switch($leve1)
　{
　　case 3:
　　　echo “高级”;
　　case 2:
　　 echo “中级”;
　 case 1:
　　　 echo “初级”;
　　default:
　　　 echo “错误的等级值”;
　}
　?>

The execution result is: Advanced Intermediate Elementary Error Level Value

What do you think of from this? ?

　<?php
　　$level = 3;
　　switch($level)
　{
　　case 3:
　　　echo “赋予管理员权限”;
　　case 2:
　echo “赋予站务权限”;
　　case 1:
　　echo “赋予版主权限”;
　　default:
　　　echo “赋予普通用户权限”;
　}
　?>

Compared with if, switch achieves higher efficiency:

　<?php
　　$a = 59;
　switch($a)
　　{
　case $a == 100;
　　echo “满分”;
　　break;
　case $a >= 90;
　　echo “优秀”;
　　break;
　case $a >= 60;
　　echo “及格”;
　　break;
　default:
　　echo “不及格”;
　}
　?>

So what is the loop statement used for? Of course it is used to perform an operation repeatedly.

While 与do···while 
While的语法： 
While(E)
　语句块;

Analysis: As long as E in the while expression is TRUE, the statement will be executed.

The syntax of do···while:

　do
　{
　　语句块;
　}
　while(E)

The difference between do···while and while is that do···while is checked at the end of the loop, regardless of whether the conditions of the loop are met or not. , do···while will be executed once.

For example:

　<?php
　　$a = 5; //先判断$a是否大于5，如果大于5则执行。
　　while($a>5)
　{
　　echo “This is while.”;
　　$a–;
　}
　do //先执行do之内的语句，然后进行判断。
　{
　　echo “This is do…while.”;
　　$a–;
　}
　while($a > 5)
　?>

For statement, syntax:

　For(A；B；C)
　　Statement;

Analysis: The first expression is at the beginning of the loop First execute unconditionally once, usually A is an assignment statement; B is run before the loop starts, if it is TRUE,

will continue to loop and execute the nested statements of the loop; C is executed after the loop, usually Self-increment and self-subtraction operations.

Code:

　<?php
　　for($a = 5;$a > 5;$a–)；
　　echo “This is for”;
　?>

Foreach statement, used for array traversal, you will learn it later.

Transfer control statements

There are three main types of transfer control statements in PHP: break, continue and return.

1. Break statement

The break statement is used to end the current loop. break can accept an optional numeric parameter to determine how many levels to jump out of. cycle.

Example:

<?php
　　$a = 5;
　　$b = 10;
　while($a <100) //$a<100开始循环
　{
　　echo “a = “.$a.”<BR>”; //输出$a，“.”时连接运算符，相当于java中的“+”
　while($b > 0) //$b>0，开始循环
　{
　　echo “b = ” .$b.”<BR>”; //输出$b
　　$b–;
　　if($b == 3 ) //如果$b==3，则跳出while($b>0)
　　break;
　}
　　$a++;
　　if($a == 30)
　　break; //如果$a==30,就跳出while($a<100)
　}
　?>

Continue statement

Continue is used to jump out of this loop, which is different from break Yes, after continue exits, it will continue to execute the next cycle.

Return statement The Return statement is used to end a function or a script file. If the return statement is called in a function, it will immediately end the execution of the function and return its value as a parameter.

Of course, return can also be used as a function in PHP. Such as return(), and write the parameters to be returned in parentheses. This usage is uncommon.

Attached is a picture for everyone to consider carefully.

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