Question about Single Dot Concatenation

Hello,
I have read the PHP Coding Style Guide page. I have a question:

Why single dot concatenation must not be separated from their subjects?
As I understand, the single dot concatenation, like other 2-ary operation, should be separated from their subjects by one space per side (excluding alignment). I think it is more pretty, more clean. Please tell my why is this convention applied?

Thanks for explanation.

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