# Composer

### 0. Introduction

`composer` is a PHP binary script and it's using [Unix Shebang](https://en.wikipedia.org/wiki/Shebang_\(Unix\)) to define which interpreter for executing.

```
root@home:~# head /usr/local/bin/composer -n 5

#!/usr/bin/env php
<?php
/*
 * This file is part of Composer.
 *


/usr/bin/php7.1 /usr/local/bin/composer update
```

### 1. Execute composer with specific PHP version

So if you're running `composer` without any specific option, it will use your PHP version at `/usr/bin/env php` (which is your default PHP)

We can have many versions of PHP inside your Linux and you can flexible use which PHP version for executing as you want like this way:

```
root@home:~# /usr/bin/php7.1 /usr/local/bin/composer update
```

Or just

```
root@home:~# php7.1 /usr/local/bin/composer
root@home:~# php7.2 /usr/local/bin/composer
root@home:~# php7.3 composer
...
```

### 2. Check & find all installed PHP packages

```
root@home:~# ls /usr/bin/ | grep php
php
php5.6
php7.1
php7.2
php7.3
php-config
php-config7.1
phpdbg
phpdbg5.6
phpize
phpize7.1
```