optimize php generator

[AxiosLeo]

1. Add debug.warning method
2. Add emitter.fixed method
3. Add test case

[codecov-commenter]

Codecov Report

Merging #49 into master will increase coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   94.97%   95.38%   +0.40%     
==========================================
  Files          15       15              
  Lines        2231     2254      +23     
==========================================
+ Hits         2119     2150      +31     
+ Misses        112      104       -8     

Impacted Files	Coverage Δ
src/generator.js	95.00% <100.00%> (+0.05%)	⬆️
src/lib/debug.js	100.00% <100.00%> (ø)
src/lib/emitter.js	100.00% <100.00%> (ø)
src/langs/php/package_info.js	85.71% <0.00%> (+16.32%)	⬆️

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