$d = date("D");
if($d == "Fri"){
    echo "Have a nice weekend!";
} else{
    echo "Have a nice day!";
}
echo "\n";

$t = date("H");
if ($t < "20") {
    echo "Have a good day!";
} else {
    echo "Have a good night!";
}

$age

18

Adult

Child

$age = 20;
if ($age > 18) {
    echo 'Adult';	
} else {
    echo 'Child';	
}

strlen()

echo strlen("Hello world!")."\n"; // 12

strrev()

echo strrev("Hello world!")."\n"; // !dlrow olleH

strpos()

echo strpos("Hello world!", "world")."\n"; // 6

str_replace()

echo str_replace("Hello", "Hi", "Hello there")."\n"; // Hi there

substr()

echo substr("Hello world", 6)."\n"; // world
echo substr("Hello world", 6, 3)."\n"; // wor
echo substr("Hello world", -5)."\n"; // world
echo substr("Hello world", -4, 2)."\n"; // or

trim()

echo trim("   Hello World  ").".\n"; // Hello World.
// Removes whitespaces from the right end
echo rtrim("   Hello World  ")."\n"; //    Hello World
// Removes Exclamation mark from the right end
echo rtrim("Hello World!", "!")."\n"; // Hello World

strtoupper()

echo strtoupper("Hello World")."\n";

strtolower()

echo strtolower("Hello World")."\n";

explode()

print_r (explode(" ", "It's the first day of the course!"));

implode()

echo implode(" ", ['Hello', 'World']);

$variable

$Variable

$VARIABLE

// lowercase
$variable = "Value 1";

// PascalCase
$Variable = "Value 2";

// UPPERCASE
$VARIABLE = "Value 3";

echo $variable . "\n";
echo $Variable . "\n";
echo $VARIABLE . "\n";

function printGreeting() {
	print "Prints: Hello world!\n";
}
printGreeting();

logError

file

line

code

message

trace

try {
    undefinedFunction();
} catch (Error $e) {
    logError(
		'ERR FILE: ' . $e->getFile() . "\n" .
		'ERR LINE: ' . $e->getLine() . "\n" .
		'ERR CODE: ' . $e->getCode() . "\n" .
		'ERR TEXT: ' . $e->getMessage() . "\n" .
		'ERR TRACE String: ' . $e->getTraceAsString()
	);
}

logError

function logError($error) {
	
	$content = "\n\n\n".date('Y-m-d H:i:s');
	$content .= "\n".$error;
	
	file_put_contents('/var/log/errors.txt', $content, FILE_APPEND);
}

int

class Book {
    public $price;
    
    public function price(int $price) {
        $this->price = $price;
    }
}

$book = new Book;

$book->price('k34');

echo $book->price;

Uncaught TypeError: Argument 1 passed to Book::price() must be of the type int, string given

class Foo {
    public function __toString()
    {
        return "Some text about the OBJECT"; 
    }
}
$foo = new Foo();
echo $foo;

__toString

interface Talkative {
    public function talk();
}
class Cat implements Talkative {
    public function talk() {
        return 'Woof' . PHP_EOL;
    }
}
class Dog implements Talkative {
    public function talk() {
        return 'Meow' . PHP_EOL;
    }
}
class Tortoise implements Talkative {
    public function talk() {
        return 'Yak yak yak yak ...' . PHP_EOL;
    }
}

$cat = new Cat;
$dog = new Dog;
$tortoise = new Tortoise;

echo $cat->talk();
echo $dog->talk();
echo $tortoise->talk();

class Bird{
    public $canFly;
    public $legCount;
    public $className;
    
    public function __construct($canFly, $legCount, $className) {
        $this->canFly = $canFly;
        $this->legCount = $legCount;
        $this->className = $className;
    }
    
    public function canFly() {
        return $this->canFly;
    }
    
    public function getLegCount() {
        return $this->legCount;
    }
    
    public function getBirdInfo() {
        $canFly = $this->canFly() ? 'can fly' : 'can not fly';
        return $this->className. ' ' . $canFly . ' and has ' . $this->getLegCount() . ' legs' . PHP_EOL;
    }
}

class Pigeon extends Bird{
    public function __construct($canFly, $legCount) {
        parent::__construct($canFly, $legCount, get_class());
    }
}

class Penguin extends Bird{
    public function __construct($canFly, $legCount) {
        parent::__construct($canFly, $legCount, get_class());
    }
}

$pigeon = new Pigeon(true, 2);
$penguin = new Penguin(false, 2);

echo $pigeon->getBirdInfo();
echo $penguin->getBirdInfo();

$search_array = array('first' => 1, 'second' => 4);
if (array_key_exists('first', $search_array)) {
    echo "The 'first' element is in the array";
}

name

gender

username

email

addFriend

deleteFriend

postStatus

follow

unfollow

class User {
    public $username;
    public $friends;
    
    public function addFriend($friend_name) {
        $this->friends[] = $friend_name;
    }
    
    public function removeFriend($friend_name) {
        $this->friends = array_filter($this->friends, fn ($m) => $m != $friend_name);
    }
}

$user1 = new User();
$user1->username = 'Adam';

$user1->addFriend('Tom');
$user1->addFriend('David');
$user1->addFriend('George');

$user1->removeFriend('David');
print_r($user1->friends);
echo $user1->username;