Warning: require(vendor/autoload.php): failed to open stream: No such file or directory in PHPMailer

PHPMailer is one of the best and most secure code library. First of all, understand the warning and fatal error in PHPMailer - 

1.Warning: require(vendor/autoload.php): failed to open stream: No such file or directory - This is not an error. It indicates that you have to install composer before using PHPMailer. You will get the vendor source after installing the composer. 

2. Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='C:xamppphpPEAR') - This is a fatal error that stops the complete execution process just because of vendor is missing in your PHPmailer.You will need a composer in your source.  

You can use the PHPMailer library with or without a composer. 

How to install composer for PHPMailer? 

To install composer, did you see a command on the same link PHPMailer

Command - 

composer require phpmailer/phpmailer

You have to install composer using this command line - 

 If you want to install composer on your computer then you have to download the composer exe file and install it. 
Before downloading composer, make sure you are using a local server or if you are working on the live server then you will not need to download composer. 
Now, run a command in the same folder where the PHPMailer folder is available. 
1.    Search run on your computer and enter cmd. 

2.    After this, a command prompt will open. 
3.    Here, you have to move the project directory. 
Like -  Commands 
cd c:\
Now you are in c drive.
cd xampp\htdocs\smtp
Here, smtp is a project folder name. You can set your project folder where the PHPMailer library is available. 
Now, run a command to install the composer 

composer require phpmailer/phpmailer

Now, you can run your PHP script without error. 

If you are unable to install composer. 

How to use the PHPMailer library without a composer? 

You can use PHPMailer library code without installing composer. 

You will need to change the code line – 

require 'vendor/autoload.php';

to 

require 'PHPMailer/src/Exception.php';
require 'PHPMailer/src/PHPMailer.php';
require 'PHPMailer/src/SMTP.php';

Now, you can use the PHPMailer library without any errors. 

You can find more information here -

How to send email using PHP Mailer library and hosting SMTP

I was also getting the same error and warning Warning: require(vendor/autoload.php): failed to open stream: No such file or directory.

Fatal error: require(): Failed opening required 'vendor/autoload.php' (include_path='C:xamppphpPEAR') . 

I was very tired to fix this error just because I was trying first time to integrate PHPMailer into the PHP website. I created an HTML form to send email using PHPMailer library but got this same error. After much research, I've got a solution on Github. You are missing composer. You have to install composer using the command line on your local host or live server where you are working. 

composer require phpmailer/phpmailer

After installing the composer, you will not get any issues with the vendor.