Calling a function with a variable variable

A way to solve this problem would be to use PHP's call_user_func function. Here is the modified code (it also removes the global variable):

Code Example

<?php

$a = 1;
$b = 1;

// I'm just using this to hold the function name,
// to get rid of the global keyword. It will be passed
// as an argument to our mainFunction()
$exampleFunction = '';

if ($a == $b) {
    $exampleFunction = 'exampleOne';
} else {
    $exampleFunction = 'exampleTwo';
}

//----------------------------------------------

mainFunction($exampleFunction);

function mainFunction($func) {
    echo 'This is mainFunction <br>';
    // Use PHP's call_user_func. We are also checking to make sure
    // the function exists here.
    if (function_exists($func)) {
        // This will call the function.
        call_user_func($func);
    }
}

function exampleOne() {
    echo 'This is example one <br>';
}

function exampleTwo() {
    echo 'This is example two <br>';
}

Output

When I run this code, it produces the following output:

This is mainFunction 
This is example two 

Try like

if ( $a == $b ) {
   $exampleFunction = exampleOne();
} else {
   $exampleFunction = exampleTwo();
}

and your functions should return like

function exampleOne() {
    return 'This is example one <br>';
}

function exampleTwo() {
    return 'This is example two <br>';
}

OR if you want to call them through the variable try to replace like

function mainFunction() {
   global $exampleFunction;
     echo 'This is mainFunction <br>';
     $exampleFunction();
 }

Try with $exampleFunction(); instead of $$exampleFunction();

OR

use call_user_func($exampleFunction)

check this way :-

function mainFunction() {
global $exampleFunction;
echo 'This is mainFunction <br>';
$exampleFunction();
 }

Use just $exampleFunction, without $$:

<?php
function mainFunction() {
    global $exampleFunction;
    echo 'This is mainFunction <br>';
    $exampleFunction();
}
?>

See manual of variable functions, not variable variables.

P.S.: Also, I suggest $exampleFunction to be an argument of mailFunction, rather than use globals.