php: how to install a package using composer - php

Question

I'm totally new to composer ;-)
I try to install mPDF 7.1 on my localhost (ubuntu 16.04) with composer.
1.) I installed composer 1.6.5.
2.) Downloaded mPDF 7.1 (https://github.com/mpdf/mpdf/releases) and unzipped folder to hdd.
3.) Open terminal in mpdf-7.1.0 folder and typed $ composer require mpdf/mpdf to start process
--> Error message is thrown:
Problem 1
- The requested package mpdf/mpdf No version set (parsed as 1.0.0) is satisfiable by mpdf/mpdf[No version set (parsed as 1.0.0)] but these conflict with your requirements or minimum-stability.
So I don't know what's the problem is and don't find any help searching the web. Can you help me?
Andreas
EDIT: I tried also:
$ composer require mpdf/mpdf:7.1.0 and
$ composer require mpdf/mpdf "^7.1.0"
throw same error message...

Answer 1

You don't need to download mPDF, that is what composer will do for you.
Do this:
Create an empty directory in which you will use mPDF
Run composer require mpdf/mpdf in the new directory
Composer will now install mPDF and create autoloading files
Require vendor/autoload.php in a PHP file
Done, you now can use mPDF in the file.

Answer 2

Make sure composer is installed globally. It works fine so.
Create an empty directory in which you will use any package, for example, mPDF
Run composer require mpdf/mpdf in the new directory
The command above does not work use sudo composer require mpdf/mpdf
If you want to specify any version of your package
Then specify thus composer require vendor/package:version
For example: composer require mpdf/mpdf:7.1.0
Composer will now install mPDF and create autoload.php in the vendor directory for autoloading files
mPDF installation is done. Now create a php file called mpdf.php for example, in same deirectory.
Require vendor/autoload.php in that PHP file.
Directory structure should be where you installed mPDF:
vendor
composer.json
composer.lock
mpdf.php
And the mpdf.php file is as follows:
<?php
require_once __DIR__ . '/vendor/autoload.php';
$mpdf = new \Mpdf\Mpdf();
$mpdf->WriteHTML('<h1>Hello world!</h1>');
$mpdf->Output();