Description:
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On the documentation page: language.operators.execution.php
I think it might be worth adding something similar to the following:

Backticks are NOT interpreted within double quotes.

Unlike the BASH shell, where:
    echo "Hello `whoami`";
will print "Hello [name]"

PHP will simply treat the backticks as literal. i.e. it prints:  
    "Hello `whoami`"

(To use backticks in PHP, do:  echo "Hello ".`whoami`;   )