Function_exists to check if function is available or not in PHP

Check for a function before calling it

function_exists() checks whether a named function is defined. It is useful for optional integrations or compatibility code, but normal application code should not need repeated checks for functions it requires.

<?php
if (function_exists('mb_strlen')) {
    echo mb_strlen('Hello');
}
?>

We can check built in function or user defined functions before using them. The built in function function_exists checks this and returns true of false. It takes one string input and returns Boolean output. Here is an example.

if(function_exists(is_int)){
echo " Yes this function is available to use ";
}else{
echo " Sorry , this is not available to use " ;
}

In the above code we have used is_int() which is a built in function in PHP, so we will get the output as this.

Yes this function is available to use

User defined functions

<?php
function my_function($var1,$var2,$var3){
$var=$var1+$var2+$var3;
return $var;
}
echo my_function(5,6,8); // Output 19 

if(function_exists(is_int)){
echo " <br>Yes <i>my_function</i> is available to use ";
}else{
echo " <br>Sorry , <i>my_function</i> is not available to use " ;
}
?>

Output

19
Yes my_function is available to use

Example of uses

To check mysqli support, function_exists() is used.

code>if (function_exists('mysqli_connect')) {
  echo "mysqli is installed";
}else{
echo " Enable Mysqli support in your PHP installation "; 
}code>

Example: Checking Custom Function

function myFunction() {
    echo "Function exists!";
}
if (function_exists('myFunction')) {
    myFunction();  // Output: Function exists!
} else {
    echo "Function not found!";
}

Example: Preventing Fatal Errors

if (function_exists('nonexistentFunction')) {
    nonexistentFunction();  // Avoids fatal error
} else {
    echo "Function does not exist!";
}

### Description:
Custom Functions: Ensures that the custom function exists before executing it. Fatal Error Prevention: Prevents runtime fatal errors by checking the function's existence. Namespaces: When working with namespaces, ensure the function exists in the correct scope. ### Example: Working with Namespaces

namespace MyNamespace;

function customFunc() {
    echo "Namespace Function";
}

if (function_exists('MyNamespace\\customFunc')) {
    customFunc();  // Output: Namespace Function
}

This way, the `function_exists()` page can be improved by providing more real-life examples and handling different PHP contexts such as namespaces and custom functions.
Declaring variables Functions

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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