I'm not sure I understand your question, so I'll pretend I do and see if I get it right.
+, -, and . all have the same precedence, and its evaluated from left to right. So if I concat inline with a + or - in it, the order changes implicitly:
$i = 4;
$j = 6;
print 'A string with ' . $i + $j . ' in it';
So the result of that would be:
print ((('A string with ' . $i) + $j) . ' in it');
Since 'A string. . .' cannot be interpreted as a number. More bizarre is what happens when you have a string that *can* be interpreted as a number:
print '2 dogs and ' . $i + $j . ' cats';
So the answer: as per any precedence control, wrap the appropriate operation in brackets to force the correct evaluation order (desired).