Search Issues (second option to the right of the search bar up top).
Some search queries you can use: 'typos', 'documentation', 'html', etc.
When you find an Issue you are interested in resolving, click on the repository name and follow the below steps:

1. Fork the repository
2. Find the ssh or https link on the right-hand side of the repository. Copy the link and then type this into your terminal:

$git clone <ssh url>

This will copy all the repository files onto your computer.
3. $cd <repo folder> 4. Make a new branch and change into it:
$git checkout -b <new branch name> 5. Open the file that contains the error in your text editor. 6. Make changes. 7. Add, commit, and push your changes.

$git add <filename>    
$git commit -m "<message>" --author-'Ellen<[email protected]>'   
$git push <remote name> <branch name>   

8. Compare and hit green 'Pull-Request' button.
9. Hit 'Create Pull Request'
10. Finished!

You can also use Code Triage to search for Github issues.
Use their 'Filter list by Language' to see PHP specific issues.