In c-era's example, he has

$funk = "cos($i)";
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This assumes that the variable $i is alrady defined. When $funk is first assigned, $i is resolved to it's value (3 in the example); so $funk actually equals "cos(3)".

If you want it dynamic, so that you can change $i and not restuff $funk, you should write it like:

$funk="cos(\$i)";
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That way you can reassign $i and just eval($funk) again to recompute it's cosine.