Detailed explanation of the difference between php die() and exit()

What is the real difference between die() and exit() in php? We found on Baidu that die exits and releases memory, and exit exits but does not release memory. Is this really the case? Friends who need it can Refer to

Search online for the difference between die and exit functions. Most of the "standard answers" say that die exits and releases memory, and exit exits but does not release memory.

This explanation is obviously wrong. The PHP manual has already said "die — Equivalent to exit(). This language construct is equivalent to exit()." The two are just aliases. Other than that, they are completely different. Same.

But I was still curious and decided to look for clues from the source code to see how PHP handles this "alias".

First of all, we must be clear that die and exit are both "language constructs" and not functions. There are many people on the Internet who say that so-and-so has a return value and is a function, and so-and-so has no return value and is a structure. Many beginners always do If you don’t know the difference between language structure and function, in layman’s terms, language structure can be understood as a symbol of grammar itself. Like, -, *, / these are also language structures, if, else, for, while, these are all language structures. is part of the grammar itself. Any language will have these things, because when the computer sees it, it will not think that it should be added. This requires a compiler to convert it into machine code, which is an instruction set that the CPU can recognize.

The entire process when PHP executes the source code is to first convert language structures such as echo and if in the source code into tokens such as T_ECHO and T_IF as defined in zend_language_scanner.l, and the source code will be removed spaces in, comment out these characters that have nothing to do with program logic. , some short expressions are formed, which is the lexical analysis stage. These tokens will then be converted into op codes as defined in zend_vm_opcodes.h. Then execute these op codes one by one.

The above roughly explains the compilation and execution process of PHP, as well as the definition of language structure. Let’s get to the point.

We should also remember that there are many alias functions in PHP, such as implode and join. Whether it is an alias function or an alias language structure, from the perspective of actual effect, they are the same, but the source code processing method is definitely different.

Let’s first look at how this alias language structure is handled, and then look at the alias function later.

zend_language_parser.c defines a macro

#define T_EXIT 300

and also defines an enum, which also contains

enum yytokentype {
…
T_EXIT = 300,
….
}

Here we are told that the T_EXIT token has a code of 300.

Look at zend_language_scanner.l, which contains these lines of code.

<ST_IN_SCRIPTING>”exit” {
return T_EXIT;
}
<ST_IN_SCRIPTING>”die” {
return T_EXIT;
}

Obviously, when php does lexical analysis, whether it encounters exit or die, it will return the T_EXIT token. From here, we can prove that die and exit, and the internal processing of PHP are exactly the same.

You can also use the following php code to determine:

<?php
var_dump(token_get_all(“<?php die;exit;?>”));

The token codes corresponding to die and exit in the returned results are both It's 300.

Now regarding the issue of die and exit, I think everyone should be able to confirm that the names are just different, but the effect is the same. There is no so-called problem of unloading or not unloading the memory.

PHP Manual: die()Equivalent to exit().

Explanation: die() and exit() are both functions to terminate script execution; in fact, the two names exit and die point to the same function, and die() is an alias of the exit() function. . This function only accepts one parameter, which can be a value returned by a program or a string, or no parameters can be entered, and the result is no return value.

Reference: Although the two are the same, there are subtle selectivity in common use. For example:
When the value passed to the exit and die functions is 0, it means that the execution of the script is terminated early, usually using the name exit().

echo "1111";
exit(0);
echo "2222";
//22222不会被输出，因为程序运行到exit(0)时，脚本已经被提前终止，“马上断气”。

When the program errors, you can pass it a string, which will be output as it is on the system terminal, usually using the name die().

$fp=fopen("./readme.txt","r") or die("不能打开该文件");

In this case, if the fopen function is called and returns the Boolean value false, die() will immediately terminate the script and immediately print the value passed to it. String, "You can say a word or two before you die."

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

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