Php to determine whether constants, variables, and functions exist _ PHP Tutorial

Php checks whether constants, variables, and functions exist. In php development, we often choose to provide predefined variables, constants, or functions to determine if they exist, next I will introduce some common judgment constants, variables, and functions to check whether the application exists. in php development, we often meet to provide pre-defined judgment variables, constants, or functions, next I will introduce some common examples of checking whether constants, variables, and functions exist in applications.

Defined is used for constant detection, and define is used for defining constants. Note that the constants to be detected must be enclosed in quotation marks (single and double quotation marks), for example:

The code is as follows:
If (defined ('const _ name ')){ // Do something }

Isset is used for variable Detection. Note that if the variable is not declared or declared, the value is NULL, and isset returns FALSE, for example:

The code is as follows:
If (isset ($ var_name )){ // Do something } Function_exists is used for function Detection. you must use quotation marks for the function name to be detected, for example: If (function_exists ('fun _ name ')){ Fun_name (); }

Let's look at an instance.

The code is as follows:
/* Determine whether a constant exists */ If (defined ('myconstant ')){ Echo MYCONSTANT; } // Determine whether a variable exists If (isset ($ myvar )){ Echo "variable $ myvar ."; } // Determine whether a function exists If (function_exists ('imap _ open ')){ Echo "the imag_openn function exists "; } Else { Echo "the imag_open function does not exist n "; } ?>

Function_exists

The code is as follows:
If (function_exists ('test _ func ')){ Echo "the test_func function exists "; } Else { Echo "the test_func function does not exist "; } ?>

Filter_has_var function

The filter_has_var () function checks whether a variable of the specified input type exists.

If yes, true is returned. otherwise, false is returned.

The code is as follows:
If (! Filter_has_var (INPUT_GET, "name ")) { Echo ("Input type does not exist "); } Else { Echo ("Input type exists "); } ?>

Output is. Input type exists

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