Description:
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The code below shows the problem. Any return statement inside the function that is supposed to create the code to be evaluated will crash PHP. Not even Apache will send a header anymore. A white page appears.

If the returned string is stored i a variable first it will work fine. The quickest fix is a (string) cast inside the eval function.

eval((string)foo());

Test script:
---------------
function foo() {
    return 'echo time();';
}

eval(foo());