Added new Singleton Design Pattern in PHP

[arturobp3]

Description

I've added a new Singleton Design Pattern in PHP

Related Issue

#427

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• [ ] Bug fix (change which fixes an issue with the algorithm)

• [x ] New Algorithm (non-breaking change which adds functionality)

• [ ] Documentation

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

• [x ] My code follows the code style of this project.

• [ ] My change requires a change to the documentation.

• [ ] I have updated the documentation accordingly.

• [ x] I have read the CONTRIBUTING document.