Detailed explanation of the allowed symbol range and rules of identifiers in PHP

What are the allowed symbols for identifiers in PHP? To explain the rules of PHP identifiers in detail, specific code examples are required

In PHP, identifiers are strings used to represent the names of variables, functions, constants, etc. Identifiers are composed of letters, numbers and underscores. The specific rules are as follows:

1. Identifiers must start with letters or underscores, and can be followed by letters, numbers or underscores.

The following are some examples of legal identifiers:

$myVariable
_alpha
myFunction
MY_CONSTANT
$myVariable_1

2. Identifiers are case-sensitive.

PHP is case-sensitive, so $myVariable and $myvariable are two different variables.

3. In PHP, reserved words used for special purposes cannot be used as identifiers.

For example, the following reserved words are keywords in PHP and cannot be used as identifiers:

if
else
for
while
foreach
function
class

These keywords have special uses and have specific syntax structures in the code.

4. There is no limit to the length of the identifier.

Identifiers in PHP can be very short or very long, with no length limit.

The following is some sample code showing legal and illegal identifiers:

// Legal identifier
$myVariable = " Hello World";
$my_function = "This is a function";
CONSTANT_NAME = "This is a constant";

// Illegal identifier
$2ndVariable = "This is invalid"; // It is illegal to start with a number
$my-variable = "This is also invalid"; // It is illegal to use a hyphen
$if = "This is invalid too"; // Use Reserved words are illegal

// Display results
echo $myVariable . "
";
echo $my_function . "
";
echo CONSTANT_NAME . "
";
?>

In the above code, $myVariable, $my_function and CONSTANT_NAME are legal identifiers and can be used normally. $2ndVariable, $my-variable and $if are illegal identifiers and cannot be used in code.

To sum up, the symbols allowed for identifiers in PHP are letters, numbers and underscores, and must start with a letter or underscore. Identifiers are case-sensitive, and reserved words cannot be used as identifiers. By following these rules, we can use legal identifiers to represent names such as variables, functions, and constants in PHP code.

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