Exemplified IF inversion technique for better readability

if-inversion-example.php

<?php
function validateApplicationTangled($user) {
    if($user->age>18) {
        if(($user->hasPreviousApplication && !$user->previousApplicationApproved) || !$user->hasPreviousApplication) {
            printf("Cool, application approved for user %s\n", $user->name);
            return true;
        } else {
            printf("Sorry, but the user %s has a previously approved application\n", $user->name);
            return false;
        }
    } else {
        printf("Sorry, but the user %s is under 18\n", $user->name);
        return false;
    }
}

function validateApplicationUntangled($user) {
    if($user->age<18) {
        printf("Sorry, but the user %s is under 18\n", $user->name);
        return false;
    }
    
    if($user->hasPreviousApplication && $user->previousApplicationApproved) {
        printf("Sorry, but the user %s has a previously approved application\n", $user->name);
        return false;
    }
    
    printf("Cool, application approved for user %s\n", $user->name);
    return true;
}

$users = [];
class User{};

$users[0] = new User();
$users[0]->age = 20;
$users[0]->name = "John";
$users[0]->hasPreviousApplication = true;
$users[0]->previousApplicationApproved = false;

$users[1] = new User();
$users[1]->age = 16;
$users[1]->name = "Emilio";
$users[1]->hasPreviousApplication = false;
$users[1]->previousApplicationApproved = null;

$users[2] = new User();
$users[2]->age = 22;
$users[2]->name = "Trajan";
$users[2]->hasPreviousApplication = true;
$users[2]->previousApplicationApproved = true;

$users[3] = new User();
$users[3]->age = 22;
$users[3]->name = "Klaus";
$users[3]->hasPreviousApplication = false;
$users[3]->previousApplicationApproved = null;

foreach ($users as $user) {
    validateApplicationTangled($user);
}

printf("\n");

foreach ($users as $user) {
    validateApplicationUntangled($user);
}