Precedence of the concatenation operator
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<?php

echo ('abc' . 1) + 2;
echo ('abc' . 1) - 2;
echo 2 + ('abc' . 1);
echo 'abc' . (1 + 2);
echo (1 + 2) . 'abc';
echo ('a' . 'b') << 1;
echo 'a' . (1 << 2);
echo 'a' . 'b' . 'c';
echo 'a' . ('b' . 'c');
echo ('a' . 'b') == 'c';
echo 'a' . ('b' == 'c');
-----
!!version=8.0
echo ('abc' . 1) + 2;
echo ('abc' . 1) - 2;
echo 2 + ('abc' . 1);
echo 'abc' . 1 + 2;
echo 1 + 2 . 'abc';
echo ('a' . 'b') << 1;
echo 'a' . 1 << 2;
echo 'a' . 'b' . 'c';
echo 'a' . ('b' . 'c');
echo 'a' . 'b' == 'c';
echo 'a' . ('b' == 'c');
-----
<?php

echo ('abc' . 1) + 2;
echo ('abc' . 1) - 2;
echo 2 + ('abc' . 1);
echo 'abc' . (1 + 2);
echo (1 + 2) . 'abc';
echo ('a' . 'b') << 1;
echo 'a' . (1 << 2);
echo 'a' . 'b' . 'c';
echo 'a' . ('b' . 'c');
echo ('a' . 'b') == 'c';
echo 'a' . ('b' == 'c');
-----
!!version=7.4
echo 'abc' . 1 + 2;
echo 'abc' . 1 - 2;
echo 2 + ('abc' . 1);
echo 'abc' . (1 + 2);
echo 1 + 2 . 'abc';
echo 'a' . 'b' << 1;
echo 'a' . (1 << 2);
echo 'a' . 'b' . 'c';
echo 'a' . ('b' . 'c');
echo 'a' . 'b' == 'c';
echo 'a' . ('b' == 'c');