Use codecov.io for gathering code coverage

[php-coder]

Second attempt. The first was in #713
Addressed to #755

[mystamps-bot]

	2 Warnings
⚠️	danger check: pull request contains 8 commits while most of the cases it should have only one. If it’s not a special case you should squash the commits into single one. But be careful because it can destroy all your changes!
⚠️	danger check: branch use_codecov does not comply with our best practices. Branch name should use the following scheme: ghXXX_meaningful-name where XXX is an issue number. Next time, please, use this scheme :)

Generated by 🚫 Danger

[codecov]

Codecov Report

Merging #714 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #714   +/-   ##
=========================================
  Coverage     70.63%   70.63%           
  Complexity      390      390           
=========================================
  Files            28       28           
  Lines          1209     1209           
  Branches        168      168           
=========================================
  Hits            854      854           
  Misses          343      343           
  Partials         12       12

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