Php checks whether constants, variables, and functions exist.

In php development, we often choose to provide pre-defined variables, constants, or functions to determine if they exist. Next we will introduce some frequently used judgment constants, variables, and functions to determine whether they exist in application instances.

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:

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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:

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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.

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<? Php /* 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

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<? Php 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.

 

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<? Php If (! Filter_has_var (INPUT_GET, "name ")) { Echo ("Input type does not exist "); } Else { Echo ("Input type exists "); } ?>

Output is. Input type exists