How safe is using .= operator on an uninitialized variable?

I'm using some premium plugins, and analyzing their code I found, that in some functions they use code like:

$output .= $some_str;

when that $output wasn't mentioned anywhere before.

How safe is this code? I tried to find any guidance in PHP Manual for this, but for what I see, they only define it for both $output and $some_str being previously set before.

Later this $output variable is used to echo HTML code.

Did you see any specifications regarding that? Maybe there is something I could do outside of those plugins to make this code safer? Some default value defined for all uninitialized variables?

Thank you!

via Chebli Mohamed