Here are some ways how we can view the .php source code for plugins:
1) via wp-admin access:
Visit the Plugins page and press the Edit link beneath the plugin name.
For example if we want to check out the bbPress source:
this link will take us to:
/wp-admin/plugin-editor.php?file=bbpress/bbpress.php
We can also visit the /wp-admin/plugin-editor.php page and select the plugin to edit.
But this will not work if we got:
define ( 'DISALLOW_FILE_EDIT', true );
in the wp-config.php file.
I don’t recommend editing plugin files through this editor. If we break something, we might not be able to access it again through the admin pages.
2) via server access:
On the server hosting our files, we most likely find the plugins under:
/path/to/your/wordpress/wp-content/plugins/
Note that it’s possible to rename the wp-content directory through code.
We might need to access the server via ftp, sftp, ssh or through some web file managers.
3) via wordpress.org:
If the plugin is avialable on wordpress.org, we can check out the developers tab.
For bbPress the path is:
https://wordpress.org/plugins/bbpress/developers/
and here’s the screenshot:
4) via GitHub/BitBucket/… repositories:
Many plugins are available on GitHub or Bitbucket. Even mirrors for the wordpress.org plugin SVN repository.
5) via the .zip file:
If you have the .zip file, you can open it and view the source files from there.
ps: bbPress was just a random plugin that I used for demonstration.