What are the rules and restrictions for PHP variables? What is the underlying principle?

The rules and restrictions for PHP variables are as follows:

1. Naming rules:

   • Variable names consist of letters, numbers, and underscores.
   • Variable names must start with a letter or underscore, not a number.
   • Variable names are case sensitive, $myVariable and $myvariable are two different variables.

2. variable type:

   • PHP is a weakly typed language, and the type of a variable is determined by its value at runtime rather than specified at declaration time.
   • Variables can be assigned values ​​of different types at any time.

3. Variable lifecycle:

   • Variables are created when they are declared and are usually destroyed at the end of the code block. The life cycle of global variables accompanies the running cycle of the entire application.

4. Variable scope:

   • PHP supports multiple scope types, including global scope, function scope, class scope, etc.
   • Variables declared inside a function have function scope and are only visible inside the function, called local variables.
   • Variables declared outside a function have global scope and can be accessed throughout the script.

The underlying principle:

1. Variable declaration: When a variable is declared in PHP code (for example: $myVariable = 10;), the PHP parser will recognize this statement and add the variable name $myVariableto the symbol table of the current scope. A symbol table is a data structure that maintains a mapping between variable names and memory addresses.

2. Variable assignment: When executing an assignment statement (for example: $myVariable = 10;), the PHP parser will find the corresponding memory address in the symbol table according to the variable name, and then store the value 10 in the memory address. Since PHP is a weakly typed language, the type of the variable will be determined here.

3. Variable use: When using a variable in the code (for example: echo $myVariable;), the PHP parser will find the corresponding memory address in the symbol table according to the variable name, and then read the value stored in the memory address, and in the corresponding Use this value in the context of .

4. Variable destruction: When a variable goes out of scope, the PHP parser removes it from the symbol table, thus destroying the variable. Local variables are destroyed at the end of the function, and global variables are destroyed at the end of the entire script execution.

In general, the underlying principles of PHP variables involve the declaration, assignment, use and destruction of variables, as well as the maintenance and management of symbol tables. These procedures enable PHP variables to be created, used and released correctly in the code.