18-Variable overview global variables linux environment variables configure java environment variables

The basic elements in modern programming languages ​​mainly include: variables, process control interfaces, functions, etc. Can I write a program without using variables? This is obviously possible, for example:

<code><?php echo "Hello NowaMagic";
?>
</code>

This program is very simple and outputs a string content.

Just like we can program using only binary, most of the work can be done without using variables. Without using variables, our program will lose great flexibility. Variables allow us to store values ​​so that they can be used in the program. Use it elsewhere, or save the new value through calculation. Variables have three basic properties:

1. Name. The identifier of the variable. Just like puppies, owners may give these puppies a favorite name. In terms of variable naming, PHP inherits the syntax style of Perl. Variables start with a dollar sign, followed by the variable name. A valid variable name begins with a letter or an underscore, followed by any number of letters, numbers, or underscores. PHP also supports composite variables, which are variables like $$a, which will be interpreted twice. This brings very flexible and dynamic features to PHP.
2. Type. The type of variable is just like the breed of a puppy. Different puppy bloodlines may be different. Some are smart, some can shop, etc. In many static languages, variables are specified when they are defined and are not allowed to be changed while the program is running. Wouldn't it be very cool if you had a puppy that could specify a breed randomly ;-) That's the case with PHP , is a weakly typed language and can be assigned any type of value.
3. Value content. This is exactly what the sign represents. It's like a real puppy. You can name any puppy: Xiaoqi. The same is true in programming languages. You can assign a value to a variable within the range it can represent. However, a variable can only have one value at the same time.

The letters that make up the variable name in PHP can be English letters a-z, A-Z, or ASCII characters from 127 to 255 (0x7f-0xff). Variable names are case-sensitive.

In addition to the variables themselves, in PHP we often come into contact with some concepts related to variables, such as: constants, global variables, static variables, type conversion, etc. In this chapter we will introduce these implementations related to variables. These include PHP's own variable low-level storage structure and the implementation of the weak type system, as well as the mutual conversion between these types.

Look at a piece of PHP code first:

<code><?php $foo = 10;
$bar = 20;

function change() {
    global $foo;
    $bar = 0;
    $foo++;
}

change();
echo $foo, ' ', $bar;
?>
</code>

Running the code will output 11 20.

But why is there such an output? How are variables implemented inside PHP? How is the scope of variables implemented? This chapter will discuss the topic of variables. Let's start with the most basic variable implementation.

Not all variables in programming languages ​​can change their values. Think about the variables we learned about in mathematics. Their values ​​are also immutable. For example: x + y = 10; The values ​​of variables x and y cannot change. In a specific scenario, that is, in a certain equation, only a specific value is represented. The advantage of the value of the variable not being changed is: This can produce as few side effects as possible. This is the case in the Erlang language, which is a functional language. Programming language is well worth learning.

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The above has introduced an overview of 18-variables, including variables and 18 aspects. I hope it will be helpful to friends who are interested in PHP tutorials.