If the execution order is defined, then there is no ambiguity about what can be run in parallel, and when they must become synchronised.
 Undefined evaluation order is a requirement for parallelization. By definition. Take for example... 
$c = $a + $b;

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If the '+' operator always evaluates the left argument before the right argument, that means they can't be run in parallel.

In reply to Re^19: Why is the execution order of subexpressions undefined? by Anonymous Monk
in thread Why is the execution order of subexpressions undefined? by BrowserUk

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