I spent years as a LISP programmer. A programmer who has memorized the precedence table will write code that's understandable by others who have memorized the precedence table. Use parenthesis liberally.

$longways = $widthIsLonger ? $width || 1 : $height || 1;
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$longways = $widthIsLonger ? ($width || 1) : ($height || 1);
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It's mostly the case that if I'm using an operator within the scope of ?: I'll parenthesize.