[PHP] re-order dependency installations in order to optimise building caches

[itscaro]

Description

When dependencies/requirements are not changed, these build steps should be cached.

Motivation and Context

• I have raised an issue to propose this change ([PHP] Optimise building cache #97)

Which issue(s) this PR fixes

Close #97

How Has This Been Tested?

Manually by changing Dockerfile

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.