fix(php): update type hint in toString method

[simPod]

If it's not datetime nor bool, it's string

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[wing328]

thanks for the PRs to improve the PHP generator.

can you please PM me via Slack to discuss and review all your PRs when you've time?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[wing328]

as discussed the type hint has been updated to include number

cc
@jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)