Rules for using PHP identifiers: analysis of symbol types and restrictions

PHP identifier usage specifications: parsing the types of symbols allowed and their restrictions, specific code examples are required

PHP is a very popular server-side scripting language. It is widely used in web development. In PHP, identifiers are names used to name entities such as variables, functions, and classes. However, there are some specifications that need to be followed when it comes to the use of PHP identifiers. This article will analyze in detail the types of symbols allowed in PHP and their limitations, and provide code examples to help readers better understand.

1. Legal PHP identifiers

In PHP, identifiers consist of letters, numbers, and underscores, and must start with a letter or underscore. Identifiers are case-sensitive, i.e. $variable and $Variable are two different variables. Here are some examples of legal PHP identifiers:

$myVariable
$MyClass
$_myFunction
$my_variable

It should be noted that this is not allowed in PHP Special symbols other than the $ symbol, such as @, !, #, etc.

2. The length limit of PHP identifiers

In PHP, the length of identifiers is limited. In PHP 7 and above, the identifier length limit for variables, classes, functions, and constants is no more than 64KB characters. If the identifier exceeds this length limit, the PHP parser will throw a ParseError error. Therefore, when writing PHP code, try to avoid identifiers that are too long to avoid errors.

3. Keywords and reserved words

In PHP, there are some reserved keywords and reserved words that cannot be used as identifiers. This is because these keywords and reserved words have special functions or meanings in PHP. Since PHP 7.4, the following is a list of keywords and reserved words reserved by PHP:

and, global, try, catch, finally, yield, self, parent, const, namespace, use, final, implements, class , function, public, protected, private, abstract, interface, trait, static, for, foreach, while, do, if, else, elseif, switch, case, default, break, continue, return, echo, print, include, include_once , require, require_once, new, clone, instanceof, throw, true, false, null, __CLASS__, __DIR__, __FILE__, __FUNCTION__, __LINE__, __METHOD__, __NAMESPACE__

Because these keywords and reserved words have special functions, Using them directly as identifiers will cause errors.

The following is a sample code that demonstrates legal PHP identifiers and illegal identifiers:

<?php

$validIdentifier = 123; // 合法的标识符
$_validIdentifier = 'valid'; // 合法的标识符
$Invalid#Identifier = 'invalid'; // 不合法的标识符，包含特殊符号#

echo $validIdentifier;
echo $_validIdentifier;
echo $Invalid#Identifier;

?>

In the above code, $validIdentifier and $_validIdentifier are legal identifiers, It works fine. The $Invalid#Identifier contains the special symbol #, which is an illegal identifier and will cause a syntax error.

Summary:

Legal PHP identifiers consist of letters, numbers, and underscores, and must start with a letter or underscore. The length of the identifier cannot exceed 64KB characters. At the same time, you need to avoid using PHP reserved keywords and reserved words as identifiers to avoid errors. When writing code, following the usage specifications of these identifiers can better write clear, readable, and maintainable PHP code.

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