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Valeri Tandilashvili's PHP Notes
Results: 133
empty vs isset
Difference between 
empty
and 
isset
$var = 0;

// Evaluates to true because $var is empty
if (empty($var)) {
    echo '$var is either 0, empty, or not set at all';
}

// Evaluates as true because $var is set
if (isset($var)) {
    echo '$var is set even though it is empty';
}
The following values are considered to be false: 
""
(an empty string) 
0
(0 as an integer) 
0.0
(0 as a float) 
"0"
(0 as a string) 
NULL
FALSE
array()
(an empty array)
Calendar CODE 

// Example usage:
$year = 2023;
$month = 1; // May

function generateMonthArray($year, $month) {
    $numDays = cal_days_in_month(CAL_GREGORIAN, $month, $year);
    $firstDay = date("N", strtotime("$year-$month-01")); // 1 = Monday, 7 = Sunday

    $weekdays = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];

    $monthArray = array_fill_keys($weekdays, []);

    for ($day = 1; $day <= $numDays; $day++) {
        $weekday = ($firstDay + $day - 2) % 7; // Adjust to start from Monday
        $monthArray[$weekdays[$weekday]][] = $day;
    }
    
    foreach ($monthArray as &$weekDays) {
    	if ($weekDays[0]!=1) {
    		array_unshift($weekDays, '');
    	} else {
    		break;
    	}
    }

    return $monthArray;
}

$result = generateMonthArray($year, $month);

// Print the result
foreach ($result as $weekday => $days) {
    echo $weekday . "\t" . implode("\t", $days) . "\n";
}
Function hoisting in PHP CODE
Functions don't need to be defined before they are called. In this example, function 
printMe()
is defined after it's called but it works without errors
echo printMe();

function printMe() {
    return 'Print some text';
}
Exception is 
conditional function
. In this example function 
foo()
will not be defined until the 
if ($makefoo)
conditional statement gets executed
$makefoo = true;

/* We can't call foo() from here 
   since it doesn't exist yet,
   but we can call bar() */
// foo();

bar();

if ($makefoo) {
  function foo()
  {
    echo "I don't exist until program execution reaches me.\n";
  }
}

/* Now we can safely call foo()
   since $makefoo evaluated to true */

if ($makefoo) foo();

function bar() 
{
  echo "I exist immediately upon program start.\n";
}
Functions within functions. Function 
bar()
will not be defined until the function 
foo()
is executed
function foo() 
{
  function bar() 
  {
    echo "I don't exist until foo() is called.\n";
  }
}

/* We can't call bar() yet
   since it doesn't exist. */
// bar();


foo();

/* Now we can call bar(),
   foo()'s processing has
   made it accessible. */

bar();
Note: All functions in PHP have the global scope - they can be called outside a function even if they were defined inside and vice versa
__call 
class Foo {
    
    public function __call($function, $arguments)
    {
        return "$function has been called with arguments: " . implode(', ', $arguments); 
    }
    
    // public function fireFunction($one, $two, $three) {
    //     return "pre-defined method";
    // }
}
$foo = new Foo();
echo $foo->fireFunction(5, 47, "third argument");
__call
is invoked when the method is inaccessible (when it's not public or does not exist at all)
store and read any property using __get and __set 
__get
and 
__set
magic methods allows us to get and set any variable without having individual getter and setter methods:
class Foo {
    protected $properties = array();

    public function __get( $key )
    {
        if(array_key_exists($key, $this->properties)){
            return $this->properties[$key];
        }
        return $this->properties[ $key ];
    }
    public function __set( $key, $value )
    {
        $this->properties[ $key ] = $value;
    }
}
$foo = new Foo();
$foo->name = 'David';

echo $foo->name;
In this example, we can set any variable, even if it does not exist. Every variable will be addd to 
properties
array
htmlspecialchars
With quotes:
$new = htmlspecialchars("<a href='test'>Test</a>", ENT_QUOTES);
echo $new; // &lt;a href=&#039;test&#039;&gt;Test&lt;/a&gt;
Without quotes:
$new = htmlspecialchars("<a href='test'>Test</a>");
echo $new; // &lt;a href='test'&gt;Test&lt;/a&gt;
The second parameter possible values: 
ENT_COMPAT
- Will convert double-quotes and leave single-quotes alone. 
ENT_QUOTES
- Will convert both double and single quotes. 
ENT_NOQUOTES
- Will leave both double and single quotes unconverted. The default is ENT_COMPAT
FILE_APPEND parameter for file_put_contents() function
Appends the content to the specified file if it exists (if the file does not exist, the function creates the file and writes the content)
file_put_contents($filePath, $content, FILE_APPEND);
Note: if 
FILE_APPEND
parameter is not passed, the function overwrites the content.
final method can not be overridden
Child class can not override final method of the parent class
class A {
    final public static function who() {
        echo __CLASS__;
    }
}

class B extends A {
    public static function who() {
        echo __CLASS__;
    }
}

B::who();
Fatal error: 
Cannot override final method A::who()
two classes extend the same abstract class CODE
Two classes extends 
Animal
abstract class and both implement the same method 
prey()
differently
abstract class Animal {
    // child classes must implement this
    abstract function prey();

    public function run() {
        echo "I am running!\n";
    }
}

class Dog extends Animal {
    public function prey() {
        echo "I killed the cat !\n";
    }
}

class Cat extends Animal {
    public function prey() {
        echo "I killed the rat !\n";
    }
}

$dog = new Dog();
$cat = new Cat();

$dog->prey(); // I killed the cat !
$cat->prey(); // I killed the rat !

$dog->run(); // I am running!
$cat->run(); // I am running!
call parent class constructor from child class CODE
We can call parent class constructor from child class using 
parent
keyword
parent::__construct($name);
Complete example:
class User {
    public $username;

    function __construct($name) {
        $this->username = $name;
    }
}
class Admin extends User{
    public $admin_level;
    
    function __construct($name, $admin_level) {
        parent::__construct($name);
        $this->admin_level = $admin_level;
    }
}
$admin1 = new Admin('Tom', 'Manager');

echo $admin1->username;
echo "\n";
echo $admin1->admin_level;
If there is no constructor in child class, then parent class constructor will be called (when appropriate parameters are passed)
Results: 133