PHP basics constant type study notes_PHP tutorial

This article will summarize some usages of PHP constant types. This is a friend’s study notes. I will share them with all my classmates.

Once a constant in PHP is defined, it cannot be changed or undefined; constants include predefined constants (internal constants) and constants defined by PHP by default. Of course, you can also customize constants.

The following are several of the predefined constants: (For complete predefined constants, please refer to the PHP manual)

TRUE, this constant is a true value (true) and is true (established).

FALSE, this constant is a false value (false), which is false (not true).

PHP_VERSION, this constant is the version number of the current PHP program, such as "5.2.17".

PHP_OS, this constant is the name of the operating system currently executing the PHP program parser, such as "Linux".

DIRECTORY_SEPARATOR, represents the path separator, which is "" on Windows and "/" on Linux.

The following are some of the PHP default constants:

E_ERROR, this constant points to the most recent error.

E_WARNING, this constant points to the nearest warning.

E_PARSE, this constant is a potential problem for parsing grammar.

E_NOTICE, this routine indicates something unusual but not necessarily an error. For example, access a variable that does not exist.

PHP provides a large number of predefined constants to any script it runs. However, many constants are defined by different extension libraries and will only appear when these extension libraries are loaded, either dynamically loaded or included at compile time.

PHP has several constants called "magic constants" whose values ​​change depending on their location in the code. For example, the value of "__LINE__" depends on the line it is located in the script. These special constants are not case-sensitive (but generally uppercase), as follows

"__LINE__" The line number in the file where this constant is located.

"__FILE__" The full path and file name of the file. If used within an included file, returns the name of the included file. __FILE__ also contains an absolute path (or the resolved absolute path in the case of a symbolic link).

The directory where the "__DIR__" file is located. If used within an included file, returns the directory where the included file is located. It is equivalent to dirname(__FILE__). Directory names do not include the trailing slash unless they are the root directory. (New in PHP 5.3.0)

"__FUNCTION__" Function name, starting from PHP 5, this constant returns the name when the function was defined (case-sensitive).

"__CLASS__" The name of the class. Since PHP 5, this constant returns the name when the class was defined (case-sensitive).

"__METHOD__" The method name of the class, returns the name when the method was defined (case-sensitive).

"__NAMESPACE__" The name of the current namespace (case sensitive). This constant is defined at compile time.

Of course, the above default constants are not enough when writing programs. The define() function allows us to define the constants we need. See example below:

1234

Constant names follow the same naming rules as any other PHP tag. Legal constant names begin with a letter or an underscore, followed by any letters, numbers, or underscores.

Constants are case-sensitive by default. By convention, constant identifiers are always uppercase and the value cannot be changed during script execution.

The difference between defining constants and defining variables:

There is no dollar sign ($) in front of the constant
Constants can only be defined using the define() function, not assignment statements
Constants can be defined and accessed anywhere regardless of variable scope rules
Once defined, a constant cannot be redefined or undefined
The value of a constant can only be a scalar