PHP learning officially sets sail (4)

Now let’s talk about functions, the functions in php are so simple

<?php
function a(){
echo "hello";
} 
a(); 
?>

function just defines a function, just like javascript, and it doesn’t require a function type, not even formal parameters. Need

<?php
$a=1; 
function a($a){
echo $a;
} 
a($a); 
?>

Output 1

<?php
$a=1; function a($a){
$a=$a+1;
echo $a;
} a($a); ?>

Output 2

<?php
$a=1; function a($a){
$a=$a+1;
echo $a;
} a($a);
echo $a; ?>

The last a is actually the beginning a. Print 1

Of course functions can have return values

<?php
function a(){
return 1;
} 
echo a(); 
?>

Print 1

In addition, PHP functions are different from other programming languages ​​in that they can be directly Assign default values ​​to formal parameters

<?php
$a=2; function add($a,$b=3){
return $a+$b;
} echo add($a); 
?>

Print 5

Isn’t this very advantageous
Of course, you can also pass other values ​​to b

<?php
$a=2; function add($a,$b=3){
return $a+$b;
} echo add($a,$b=5); 
?>

Print 7

Let’s briefly talk about the introduction of php
This concept has been in the C language as early as the introduction of header files
php has two functions that can introduce files
include The English meanings of () function and require() function are inclusion and introduction respectively, which have similar meanings.
The functions of the two functions are actually the same, but there is a little difference
include() function. If an error occurs (for example, the imported file does not exists), the code will still be executed later
require() function once an error occurs, the following code will no longer be executed, that is, the program will be interrupted

See below
Create a new php file a.php in The root directory
code is as follows

<?php
echo "hello";
?>

is very simple

Then another php file is introduced

<?php
include &#39;a.php&#39;;
echo "hello";
?>

will print two hellos

If the file does not exist

<?php
include &#39;a1.php&#39;;
echo "hello";
?>

there will be a warning, but the second hello will still be printed

and

<?php
require &#39;a1.php&#39;;
echo "hello";
?>

will report a fatal error, the second hello will be printed as usual hello will not print

In addition, require ('a.php'); can also be added with or without parentheses, and quotation marks can also be double quotes or single quotes

In addition, include_once( ) and require_once()
The function is the same as include and require, but it will determine whether the file has been introduced before. If it has been introduced before, it will not be introduced

<?php
require (&#39;a.php&#39;);
require (&#39;a.php&#39;);
?>

Print two hellos

<?php
require (&#39;a.php&#39;);
require_once (&#39;a.php&#39;);
?>

Print one hello

Even if it is preceded by include

<?php
include (&#39;a.php&#39;);
require_once (&#39;a.php&#39;);
?>

Print one hello

include_once() has the same function as above

require_once is generally used, especially in large projects
include usually introduces the head and tail html code of html, and rarely introduces php The code
require is to introduce a class or function

In fact, if you are confident enough to confirm that the same file will not be introduced more than once, it is completely fine to use require, and the efficiency is higher, at least no judgment is needed , but it should be used with caution in large projects. Because it is a team collaboration, the code written by others may introduce the files you want to introduce, and it is impossible to check them one by one, so it is appropriate to use require_once

In addition, Let me talk about a small problem. When introducing a pure PHP file (without HTML code), it is best for the imported file not to have a closing tag?>

In fact, PHP code (without HTML)

<?php
echo "hello";

is executable. If you don’t write the system and read the document, you will find that it will not add a

by default. Why is this?
Because generally speaking

<?php
echo "hello";
?>

then another php introduction

<?php
require (&#39;a.php&#39;);
echo "hello";
?>

will print hellohello

but if you are not careful

<?php
echo "hello";
?> ------------光标在>后面时候你打个几个回车
---------这里有回车符号了，不过你看不到
------------回车符号
-------------

Most people have this habit, right

Then introduce printing

<?php
require (&#39;a.php&#39;);
echo "hello";
?>

will print
hello hello will be separated in the middle If there is a space

without adding ?>, this situation will not happen

So. . Unless you ensure that there are no symbols (carriage returns, spaces, etc.) behind the closing tag?>
You can use the closing tag
In fact, many projects are now aware of this. It is basically rare to use the closing tag when writing classes

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