x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


<?php
function eval_with_x($code, $a, $b) {
    $x = $a;
    $first = eval($code);
    $x = $b;
    $second = eval($code);
    return $second - $first;
}
 
echo eval_with_x('return 3 * $x;', 5, 10), "\n"; # Prints "15".
?>