PHP basic interview questions

Hypertext Preprocessor (PHP) is server side scripting language for web development. It was created by Rasmus Lerdorf in 1994. the PHP reference implementation is now produced by The PHP Group

They both are used to print values. However print_r() is used to print an array,object while echo is used to print only single values like variables, constants etc.

$num_arr = [1,2,3,4];
echo $num_arr; // prints only Array
print_r($num_arr); // prints Array(1,2,3,4)

They all are used to include files.

Only difference between require and include is about the error they produce. require() gives fatal error if the file is not found and stop the script execution but include() gives a warning but the script continue to execute.

require() and require_once() are identical but require_once is used to check if the file is already included on same page or not. If it is not included, it includes the file otherwise skip to add those file.

In GET method , data is sent via url

<form action = "<?php $_PHP_SELF ?>" method = "GET">          
   <input type = "text" name = "name" placeholder="Enter Your Name"/>         
   <input type = "text" name = "age" placeholder="Enter Your Age"/>          
   <input type = "submit" />       
</form> 

value is passed using page_url?name=curious&age=27

this value can be accessed using $_GET[”]

 <?php   
 if( $_GET["name"] || $_GET["age"] ) {      
     echo "Name ". $_GET['name'];       
     echo "Age ". $_GET['age'];              
} 
?> 

GET method is used when data sent is small like for search, filtration etc. Multimedia can not be sent via GET Method.

POST method : Data is sent via HTTP headers. There is no restriction on the size of data.

 <form action = "<?php $_PHP_SELF ?>" method = "POST">          
   <input type = "text" name = "name" placeholder="Enter Your Name"/>         
   <input type = "text" name = "age" placeholder="Enter Your Age"/>          
   <input type = "submit" />       
</form>  

<?php   
 if( $_GET["name"] || $_POST["age"] ) {      
     echo "Name ". $_POST['name'];       
     echo "Age ". $_POST['age'];              
} 
?>  

using define()

Syntax

 <?php define(name, value, case-insensitive); ?> 

By default case-sensitive value is false

Example

 <?php define("sayHello", "Hello!!"); ?>
 <?php define("sayHello", "Hello!!", true);  ?>

Cookie is a small file that is stored at client browser.

To create a cookie in PHP

<?php setcookie(name, value, expire); ?>

Example:

 <?php setcookie("Name", "John", time()+1*60*60); ?>

Here time is optional

To get a value of cookie

<?php $_COOKIE["Name"]; ?>

Session used to stores files on server temporarily. The location of the temporary file is determined by a setting in the php.ini file called session.save_path.

To create a session , first start a session by using session_start()

To set a session

 <?php $_SESSION['name'] = 'john'; ?>

Get session value

 <?php if( isset( $_SESSION['name'] ) ) { echo  $_SESSION['name']; } ?>

Unset Session value

 <?php unset($_SESSION['name']); ?> 

To destroy all session values

 <?php session_destroy(); ?>

using mysqli_connect()

<?php
// connection to the database
$servername = "localhost";
$username = "username";
$password = "password";
//object oriented way
$conn = new mysqli($servername, $username, $password);
// procedural way
//mysqli_connect(host, username, password, dbname, port, socket)
  
// Check connection
if ($conn->connect_error) {
   die("Connection failed due to : " . $conn->connect_error);
} else{
echo "Connecton Successful";
}
 ?>   

using unlink() function

 <?php unlink("test.txt"); ?>

They both are used to store information. But here is the difference :

To upload a file

<?php  move_uploaded_file($file, $dest) ?>

File can be download via different methods

1. Using file_get_content()

Here file_get_content first download the file and using file_put_content it saves the file.

 <?php file_put_contents( $file_name,file_get_contents($url_of_file)); ?> 

2. Using curl

 <?php 
  
 // Initialize a file URL to the variable 
 $url = 'https://curiouswebs.in/wp-content/uploads/2019/12/women.png'; 
  
 // Initialize the cURL session 
 $ch = curl_init($url); 
  
 // Inintialize directory name where 
 // file will be save 
 $dir = './'; 
  
 // Use basename() function to return 
 // the base name of file  
 $file_name = basename($url); 
  
 // Save file into file location 
 $save_file_loc = $dir . $file_name; 
  
 // Open file  
 $fp = fopen($save_file_loc, 'wb'); 
  
 // It set an option for a cURL transfer 
 curl_setopt($ch, CURLOPT_FILE, $fp); 
 curl_setopt($ch, CURLOPT_HEADER, 0); 
  
 // Perform a cURL session 
 curl_exec($ch); 
  
 // Closes a cURL session and frees all resources 
 curl_close($ch); 
  
 // Close file 
 fclose($fp); 
  
 ?>  

An array is a collection of similar items stored in a variable. Array index always starts from 0.

Example of simple array in PHP

 <?php
 $fruits = array("Banana", "Apple", "Orange");
 echo $fruits[0], $fruits[0],  $fruits[0]."are my favorite fruits."
 ?>   

In PHP, there are three types of arrays.

• Indexed arrays – Arrays with a numeric index
• Associative arrays – Arrays with named keys
• Multidimensional arrays – Arrays containing one or more arrays

Indexed Arrays

<?php
 $fruits = array("Banana", "Apple", "Orange");
 for($i = 0; $i < $arrlength; $i++) {
  echo  $fruits[$i]."<br/>";
} 
 ?>   

Associative arrays

<?php
 $person = array("fname" => "John", "lname", "Cena");
 $person['age'] = 35; // add values too array
// access values
foreach( $person  as $key => $value) {
 echo $key." : ". $value."<br/>;
} 
?>   

Multidimensional Array

$posts = array(
   array("First Post","lorem ipsum dummy content"),
   array("Second Post","lorem ipsum dummy content"),
   array("Third Post","lorem ipsum dummy content")
);  

Using php mail() function.

<?php
$to = "xyz@yopmail.com";
$subject = "Test Email";
$msg = "This is test email. Kindly ignore";
mail($to,$subject,$msg);
?>

Headers can also be modified

<?php
$to = "xyz@yopmail.com, abc@yopmail.com";
$subject = "Test Email";
$msg = "This is test email. Kindly ignore";
$header = "From:abc@somedomain.com \r\n"; 
$header .= "Cc:afgh@somedomain.com \r\n"; 
$header .= "MIME-Version: 1.0\r\n"; 
$header .= "Content-type: text/html\r\n"; 
mail($to,$subject,$msg,$headers);
?> 

using php command

 C:\xampp\htdocs>php test.php 

It is predefined function in php used to set or modify HTTP headers values

For example to redirect user from one page to another.

<?php
header('Location: http://www.example.com/');
?> 

set type

 <?php header('Content-Type: application/pdf'); ?>

set cache directives

<?php
header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past
?>       

using explode function

<?php explode(' ',"this is a string"); ?>

The PHP Data Objects (PDO) defines a lightweight interface for accessing databases in PHP. It provides a data-access abstraction layer for working with databases in PHP. It defines consistent API for working with various database systems.

 <?php
$servername = "localhost";
$username = "admin";
$password = "admin_123";

try {
  $conn = new PDO("mysql:host=$servername;dbname=myDB", $username, $password);
     // set the PDO error mode to exception
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
     echo "Connection success";  
} catch(PDOException $e) {
  echo "Connection failed: " . $e->getMessage();
}
?>   

These are PHP predefined variables also called superglobals as they are accessible everywhere i.e. in any class , function, file etc.

preg_match() function searches string for pattern, returning true if pattern exists, and false otherwise.

<?php
preg_match('/foo/', 'foobarbaz', $matches, PREG_OFFSET_CAPTURE);
print_r($matches);
?>               

preg_split() function is used to convert a string into an array.

<?php 
$ip = "123.456.789.000";    
$iparr = preg_split ("/\./", $ip); 
print_r($iparr);
?>

<?php
class Person{
public $fname;
public $lname; 
function set_name($fname, $lname){
 $fname = $this->fname;
 $lname = $this->lname ;
}
function get_name(){
 return $this->fname." ".$this->lname ; 
}
}
$person = new Person(); //creating object : instance of class
$person.set_name("curious","webs"); // accessing member function
echo $person.get_name();

Constructor : It is used to initialize object properties at the time of creation of object.

<?php
class Person{
public $fname;
public $lname; 
function  __construct($fname, $lname){
 $fname = $this->fname;
 $lname = $this->lname ;
}
function get_name(){
 return $this->fname." ".$this->lname ; 
}
}
$person = new Person( "curious","webs"); 
echo $person.get_name(); 

Destructor : It is used to destruct the object properties. This function is automatically called at the end of the script.

<?php
class Person{
public $fname;
public $lname; 
function  __construct($fname, $lname){
 $fname = $this->fname;
 $lname = $this->lname ;
}
function __destruct(){
 echo $this->fname." ".$this->lname ; 
}
}
$person = new Person( "curious","webs");  

public : the property or method can be access from anywhere. By default properties and methods are public.

protected : the property or method can e accessible by class itself or classes inherited from it.

private : only member of the class can access its data.

<?php
class Person {
  public $name;
  protected $age;
  private $sex;
}

 $person = new Person();
 $person ->name = 'John'; 
 $person ->age = '30'; // fatal error
 $person ->sex = 'Female'; // fatal error
?>   

The process of acquring parent class properties into child class is called inheritance.

Using extend keyword

 <?php     
class Vehicle {        
 // parent class code   
}          
class Car extends Vehicle {         
  // child class code     
}          
class Ford extends Vehicle {         
// child class code    
} 
?> 

Click here more info on PHP classes and inheritance.

PHP does not support multiple inheritance. PHP supports multileve inheritance. Child class class can’t inherit by more than one parent class.

class A { // body } 
class B { // body } 
class C extends B { // body } // will not work

//Multilevel Inheritance
class A { // body } 
class B  extends  A { // body } 
class C extends  B { // body }

If a function is defined with final keyword it can not be override. Same if class is declared final itself , it cannot be inherited to some another class.

<?php
class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }
   
   final public function moreTesting() {
       echo "BaseClass::moreTesting() called\n";
   }
}

class ChildClass extends BaseClass {
   public function moreTesting() {
       echo "ChildClass::moreTesting() called\n";
   }
}
// Results in Fatal error: Cannot override final method BaseClass::moreTesting()
?>      

Function Overloading : When two or more function have same name but different number of arguments is called function overloading.

This can be achieved in PHP via _call() magic method that takes function name and arguments

<?php
class Person{
function _call($func_name,$arguments){
if($func_name == "name"){
switch(count($arguments)){
case 0 : return;
case 1 : return  $arguments[0];
case 2 : return  $arguments[0] . " " .  $arguments[1];
}
}
}
}
$person = new Person();
echo $person->name("curious");
echo $person->name("curious","webs");

Function Overriding : It is used to override parent class function. In this both parent and child class have same function name and same number of arguments.

class Person{
public function name(){
echo "John";
}
}
class Male extends Person(){
public function name(){
echo "John"." Cena";
} 
$person = new Person();
$person->name(); // John
$male = new Male();
$male->name(); // John Cena
}

XSS attacks can be prevented by
1) Data Validations : By proper validating the data. For example password must be at least 8 character long, having one special character, capital letter.
2) Data Sanitization : Data can be sanitized before storing into the database. For example for name value make sure there is no html tag associated with it. Using strip_tags remove all html tags from it.

For more information about xss attacks check this article here.

returns the type of image.

<?php
if (imagetypes() & IMG_PNG) {
    echo "PNG Support is enabled";
}
?>       

The way by which PHP assign type to its variable is called type casting.

$str = "10"; //type is string
$str = (boolean) $str; // type is now boolean

Now, a variable’s type is determined by the context in which the variable is used. So, the type of the variable is changed automatically based on the value assigned to it. This is called Type juggling.

By default variables are passed by value in PHP. However to pass by reference i.e. to make changes in argument use & sign before the variable.

<?php function add_some_extra(&$string) {    
 $string .= ' adding extra string'; 
} 
$str = 'Hello'; 
add_some_extra($str);
echo $str;     
?> 

php.ini file contains php configuration settings. This file is located at the root of site. Each time PHP is initialized, the php.ini file is read by the system.

Some Basic Settings in php.ini file

No, PHP is not strongly typed language. It is loosely type language i.e variable declaration does not required before using it. On the other hand like C# are strongly typed language, a variable must be declared before using.

empty returns true if the variable is empty i.e. ” “(empty string). But isset() checks whether the value is set ot not. It returns true if value is set other than NULL.

It is used to dump information about the variables. It display variable’s type and value.

For single line

/**
*** Multi line comment
**/
$a = 10; // single line comments

HTML5 provides three methods of encoding

break statement is used to terminate a block and gain control out of the loop or switch, whereas continue statement will not terminate the loop but promotes the loop to go to the next iteration.

continue statement can not be implemented in switch. break statement can be used in any for, while, do-while, switch statement

Namespace are used to define same class name or function name in one program.

 <?php 
 namespace MyNamespaceName; 
 function hello()  
 { 
 echo 'Hello I am Running from a namespace!'; 
 } 
  
 // Resolves to MyNamespaceName\hello 
 hello(); 
  
 // Explicitly resolves to MyNamespaceName\hello 
 namespace\hello(); 
 ?>  

By using $SERVER[“PHP_SELF]

It is php library used to download data over FTP or HTTP.

$ch = curl_init("https://curiouswebs.in");
 $fp = fopen("index.php", "w");
 curl_setopt($ch, CURLOPT_FILE, $fp);
 curl_setopt($ch, CURLOPT_HEADER, 0);
 curl_exec($ch);
 curl_close($ch);
 fclose($fp); 

$$var is a normal variable. $$var is a reference variable