[Slim] Add PHPLint tool to check syntax

[ybelenko]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

PHPLint is a tool that can speed up linting of php files by running several lint processes at once.

Ref: overtrue/phplint

$ composer run phplint
> phplint ./ --exclude=vendor
phplint 1.0.2 by overtrue and contributors.

No config file loaded.

.......................................     40 / 87 (45%)
........................................     80 / 87 (91%)
........

Time: 3 secs    Memory: 2.0 MiB Cache: No

OK! (Files: 87, Success: 87)

Upgrade note

We have to increase required PHP version to 5.5.9, because it's minimum version for PHPLint tool.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh

[wing328]

We have to increase required PHP version to 5.5.9, because it's minimum version for PHPLint tool.

@ybelenko Given that it's a breaking change with fallback (as you've labeled), please file the PR against 3.3.x instead (you can update this PR by editing the subject line, and you will see a drop-down menu to change the target branch)

[ybelenko]

@wing328
What if I've based it on current master branch and not 3.3.x. Do I need to rebase it on 3.3.x and check everything again?

[wing328]

We sync changes from master to 3.3.x and then from 3.3.x to 4.0.x from time to time so you shouldn't need to rework this PR.

Please give it a try and you can always change it (target branch) to master if you want to.

[wing328]

LGTM 👍

[wing328]

@ybelenko thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544