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:

<?php 
/* 判断常量是否存在*/ if (defined(&#39;MYCONSTANT&#39;)) { 
echo MYCONSTANT; 
} 
//判断变量是否存在 if (isset($myvar)) { 
echo "存在变量$myvar."; 
} 
//判断函数是否存在 if (function_exists(&#39;imap_open&#39;)) { 
echo "存在函数imag_openn"; 
} else { 
echo "函数imag_open不存在n"; 
} 
?>

function_exists determines whether the function exists

1 <?php
2 if (function_exists(&#39;test_func&#39;)) {
3     echo "函数test_func存在";
4 } else {
5     echo "函数test_func不存在";
6 }
7 ?>

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 <?php 
2 if(!filter_has_var(INPUT_GET, "name")) 
3  { 
4  echo("Input type does not exist"); 
5  } 
6 else 
7  { 
8  echo("Input type exists"); 
9  }
10 ?>

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