Commonly confused knowledge points in PHP

This article mainly introduces the knowledge points that are often confused in PHP. Interested friends can refer to it. I hope it will be helpful to everyone.

1. The difference between echo and print

The functions of echo and print in PHP are basically the same (output), but there are still subtle differences between the two. There is no return value after echo output, but print has a return value, and it returns false when its execution fails. Therefore, it can be used as a normal function. For example, after executing the following code, the value of variable $r will be 1.

$r = print "Hello World";

This means that print can be used in some complex expressions, but echo cannot. However, because the echo statement does not require any value to be returned, the echo statement in the code runs slightly faster than the print statement.

2. The difference between include and require

The functions of include() and require() are basically the same (include), but there are some differences in usage. include () is a conditional inclusion function, while require() is an unconditional inclusion function. For example, in the following code, if the variable $a is true, the file a.php will be included:

if($a){   
include("a.php");   
}

And require() is different from include(). No matter what value $a takes, the following code will Include the file a.php into the file:

if($a){   
require("a.php");   
}

In terms of error handling, use the include statement. If an include error occurs, the program will skip the include statement. Although the error message will be displayed, the program will continue to execute! But require will give you a fatal error.

Of course, we can also understand Qifen literally: require means a very strong request or requirement.

3.require_once() and include_once() statements

I’m off topic, because they look similar, the simple require_once() and include_once() statements respectively correspond to require () and include() statements. The require_once() and include_once() statements are mainly used when multiple files need to be included, which can effectively avoid the error of repeated definition of functions or variables caused by including the same piece of code.

4. The difference between empty string ('') and NULL

Empty strings and NULL in PHP are both stored with a value of 0, but their types It's not the same. You can try echo gettype(''); and echo gettype(NULL); and you will find that they print string and NULL respectively. Of course, 0 is also easy to confuse. You can try echo gettype( 0); If you print the type, you will find that the type of 0 is integer (integer). It can be seen that string (''), NULL and 0 are "equal values" but not unequal types.

5. The difference between isset and empty

We can understand from the literal meaning: empty is to judge whether a variable is "empty", while isset is to judge whether Whether a variable has been set. But there is one thing you must pay attention to here: when the value of a variable is 0, empty considers the variable to be equal to empty, that is, it is equivalent to no setting. For example, when we detect the $id variable, when $id=0, we use empty and isset to detect whether the variable $id has been configured. Both will return different values: empty thinks it is not configured, and isset can get the value of $id. , look at the example below:

$id=0;   
empty($id)?print "我是空的":print "我是$id ."; //结果：我是空的   
!isset($id)?print "我是空的":print "我是$id .";//结果：我是0

6. The difference between == (equal) and === (equal)

Review the fourth empty string above ( "") and NULL, let's look at another example:

'' == NULL;   
'' === NULL;

After running, you will find that the first one is true, and the second one is false! It can be seen that == only compares whether the values ​​are equal, while === not only compares the values, but also compares the types, which is more strict.

7.self :: The difference between this->

When accessing member variables or methods in a PHP class, if the referenced variable or method is If it is declared as const (defining constants) or static (declaring static), then the operator:: must be used. On the contrary, if the referenced variable or method is not declared as const or static, then the operator -> must be used.

In addition, if you access a const or static variable or method from inside the class, you must use self-reference. On the contrary, if you access a non-const or static variable or method from inside the class, you must use Self-referential $this.

8. The difference between strstr() and strpos()

stristr() is not case-sensitive strstr() is case-sensitive

Function search The first occurrence of a string within another string.

If successful, returns the remainder of the string (from the point of the match). If the string is not found, returns false.

stripos() is not case-sensitive strpos() is case-sensitive

The function returns the position of the first occurrence of a string in another string.

If the string is not found, return false.

Tests have proven that if you just search to determine whether it exists, the execution efficiency of strpos() is greater than strstr()

9. HTTP_HOST and SERVER_NAME in PHP

Same points:

When the following three conditions are met, both will output the same information.

1. The server is port 80

2. The ServerName setting in apache’s conf is correct

3. HTTP/1.1 protocol specification

is different Point:

1. Normal situation:

_SERVER["HTTP_HOST"] Under the HTTP/1.1 protocol specification, information will be output according to the client's HTTP request.

_SERVER["SERVER_NAME"] By default, the ServerName value in the apache configuration file httpd.conf is directly output.

2. When the server is not port 80:

_SERVER["HTTP_HOST"] will output the port number, for example: mimiz.cn:8080

_SERVER["SERVER_NAME "] will directly output the ServerName value

, so in this case, it can be understood as: HTTP_HOST = SERVER_NAME : SERVER_PORT

3. When the ServerName in the configuration file httpd.conf is the same as HTTP/1.0 When the requested domain name is inconsistent:

httpd.conf is configured as follows:

ServerName mimiz.cn

ServerAlias ​​www.mimiz.cn

Client access domain name www.mimiz.cn

_SERVER["HTTP_HOST"] output www.mimiz.cn

_SERVER["SERVER_NAME"] output mimiz.cn

So, in the actual program , you should try to use _SERVER["HTTP_HOST"], which is safer and more reliable.

If you are using port mapping and accessing from the intranet, it is better to use "$_SERVER['HTTP_X_FORWARDED_HOST']".

Summary: The above is the entire content of this article, I hope it will be helpful to everyone's study.

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