What does this return?


<?php
$a 
3$b 7;
echo 
"sum: " $a $b;
?>

In PHP 7.4:


The behavior of unparenthesized expressions containing both '.' and '+'/'-'
    will change in PHP 8: '+'/'-' will take a higher precedence in example.php on line 3

The impact to existing open-source code is de facto invisible; all found occurrences are actual bugs. This is a strong indicator that the overall impact will also be very minimal.