PHP determines whether a constant exists

php determines whether constants, variables and functions are functions

Determine whether constants are defined:

if (defined('CONST_NAME')) {
     //do something 
}

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

if (isset($var_name)) {
     //do something
}

Use function_exists for function detection. Note that the function name to be detected also needs to use quotation marks, such as:

if (function_exists('fun_name')) {
  fun_name();
}

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

<?php
if (function_exists(&#39;test_func&#39;)) {
    echo "函数test_func存在";
} else {
    echo "函数test_func不存在";
}
?>

filter_has_var function

filter_has_var() function checks whether the specified function exists Variable of input type. Returns true if successful, false otherwise.

<?php
if(!filter_has_var(INPUT_GET, "name"))
 {
 echo("Input type does not exist");
 }
else
 {
 echo("Input type exists");
 }
?>

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