php check if variable exists

PHP Determines whether constants, variables and functions exist

Determines whether variables are defined: defined()

1 if (defined('CONST_NAME')) {
2     //do something 
3 }

Determine whether the variable exists: isset(). Note that the variable is not declared or is assigned a value of NULL when declared. isset returns FALSE, such as:

1 if (isset($var_name)) {
2     //do something
3 }

Function detection Use function_exists, note that the function name to be detected also needs to use quotation marks, such as:

1 if (function_exists('fun_name')) {
2  fun_name();
3 }

Example:

function_exists determines whether the function exists

1 

filter_has_var function

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

If successful, return true, otherwise return false.

1 

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