// The array_combine() function creates an array by using the elements from one "keys" array and one "values" array.

$firstName=["gela", "bela", "lela"];
$age = [18, 19, 20];
$keyValue = array_combine($firstName, $age);
print_r($keyValue);
// ( "gela" => 18,  "bela" => 19, "lela" => 20 )

// Please note that both arrays should have equal quantity of elements !!!

Check whether a variable is empty. Also check whether the variable is set/declared:

<?php
$a = 0;
// True because $a is set
if (isset($a)) {
  echo "Variable 'a' is set.<br>";
}

$b = null;
// False because $b is NULL
if (isset($b)) {
  echo "Variable 'b' is set.";
}
?>

➤ The exit() function in PHP is an inbuilt function which is used to output a message and terminate the current script. • The exit() function returns nothing. • The exit() function only terminates the execution of the script. For instance:

$x = 10;
$y = 5;
exit; // Terminates the execution of the script
echo $x + $y; // This line will not be executed

$x = 10;
$y = 5;
exit("terminate the current script"); // Output a message
echo $x + $y; // This line will not be executed

➤ BubbleSort Algorythm.

function bubbleSort(&$array)
{
	if ($length = count($array)) {
		for ($i = 0; $i < $length; $i++) {
			for ($k = 0; $k < $length; $k++) {
				if ($array[$i] < $array[$k]) { // > descending
					$tmp = $array[$i];
					$array[$i] = $array[$k];
					$array[$k] = $tmp;
				}
			}
		}
	}
}
$arr = [2,99,15,72,14,1,26,923,34,66,7];
bubbleSort($arr);
print_r ($arr); // outputs: 1 2 7 14 15 26 34 66 72 99 923

➤ InsertSort Algorythm.

function insertSort(&$array) {
	if ($length = count($array)) {
		for ($i = 0; $i < ($length - 1); $i++) {
			$key = $i + 1;
			$tmp = $array[$key];
			for ($k = ($i + 1); $k > 0; $k--) {
				if ($tmp < $array[$k - 1]) { // > descending
					$array[$k] = $array[$k - 1];
					$key = $k - 1;
				}
			}
		        $array[$key] = $tmp;
		}
	}
}
$arr = [2,99,15,72,14,1,26,923,34,66,7];
insertSort($arr);
print_r ($arr); // 1 2 7 14 15 26 34 66 72 99 923

➤ sort() - sort arrays in ascending order.

$numbers = array(5, 1, 3, 4, 2);
sort($numbers);
print_r ($numbers); // outputs:  1 2 3 4 5

$cars = array("Ferrari", "BMW", "Mustang");
sort($cars);
print_r ($cars); // outputs:  BMW Ferrari Mustang
// ASCII Code 66 - B
// ASCII Code 70 - F
// ASCII Code 77 - M

➤ rsort() - sort arrays in descending order.

$numbers = array(5, 1, 3, 4, 2);
rsort($numbers);
print_r ($numbers); // outputs:  5 4 3 2 1

➤ asort() - sort associative arrays in ascending order, according to the value.

$age = array("Keira Knightley"=>"37", "Mads Mikkelsen"=>"56", "Keanu Reeves"=>"58");
asort($age);
foreach($age as $key => $value) {
  echo "Key = $key -> Value = $value \n";
}
// outputs:
// Key = Keira Knightley -> Value = 37 
// Key = Mads Mikkelsen -> Value = 56 
// Key = Keanu Reeves -> Value = 58

➤ ksort() - sort associative arrays in ascending order, according to the key.

$age = array("Keira Knightley"=>"37", "Mads Mikkelsen"=>"56", "Keanu Reeves"=>"58");
ksort($age);
foreach($age as $key => $value) {
  echo "Key = $key -> Value = $value \n";
}
// outputs:
// Key = Keanu Reeves -> Value = 58 
// Key = Keira Knightley -> Value = 37 
// Key = Mads Mikkelsen -> Value = 56 

• arsort() - sort associative arrays in descending order, according to the value. • krsort() - sort associative arrays in descending order, according to the key.

➤ The array_count_values() function counts all the values of an array. Syntax:

array_count_values(array)

For instance:

$animals = array(
	"Cat",
	"Cat",
	"Dog",
	"Rabbit",
	"Rabbit",
	"Dog",
	"Dog",
	"Dog",
	"Cat",
	);
print_r (array_count_values($animals));
/* outputs: 
Array
(
    [Cat] => 3
    [Dog] => 4
    [Rabbit] => 2
)
*/

➤ The unset() function unsets a variable. Syntax:

unset(variable, ....); //  second parameter is optional, another variable to check

For instance:

$x = 10;
unset($x);
// False because $x is NULL
if (isset($x)) {
  echo "Variable 'x' is set\n";
}

// Declare an array
$array = array('PHP'=> true);
unset($array['PHP']); 
echo isset($array['PHP']) ? 'Array is set.' : 
    'Array is not set.';
// outputs:  Array is not set.

➤ The isset() function checks whether a variable is set, which means that it has to be declared and is not NULL. This function returns true if the variable exists and is not NULL, otherwise it returns false. Syntax:

isset(variable, ....); //  second parameter is optional, another variable to check

For instance:

$x = 0;
// True because $x is set
if (isset($x)) {
  echo "Variable 'x' is set\n";
}
$y = null; // or $y;
// False because $y is NULL
if (isset($y)) {
  echo "Variable 'y' is set.";

// Declare an array
$array = array('PHP'=> true);
  
// Use isset function
echo isset($array['PHP']) ? 'Array is set.' : 
    'Array is not set.';
//  outputs:  Array is set.
}

➤ By default, PHP variables are passed by value as the function arguments in PHP. When variables in PHP is passed by value, the scope of the variable defined at function level bound within the scope of function. Changing either of the variables doesn’t have any effect on either of the variables. ➤ When variables are passed by reference, use & (ampersand) symbol need to be added before variable parameter. For example: function( &$x ). Scope of both global and function variable becomes global as both variables are defined by same reference. Therefore, whenever global variable is change, variable inside function also gets changed. For instance:

function print_string( &$str ) {
    $str = " PHP \n";
    print( $str );
}
$string = " JS \n";
print_string( $string ); // outputs:  PHP
print( $string ); // outputs:  PHP

➤ In PHP, References enable accessing the same variable content by different names. A reference variable is created by prefixing & (ampersand) sign to original variable. For instance:

$x = 10;
$y = 20;
$y = &$x;

echo " x = $x \n"; // outputs:  x = 10 
echo " y = $y \n\n"; // outputs:  y = 10 

$y = 20;

echo " x = $x \n"; // outputs:  x = 20 
echo " y = $y \n"; // outputs:  y = 20