Add AmbiguousReturnCheck

[wata727]

For example, strpos returns false when a second argument string is not included in a first argument.

<?php

strpos("abc", "d"); # => false

However, this function may return falsy value. The example is as follows:

<?php

strpos("abc", "a"); # => 0

In other words, unless you use the identical operator, you may get unintended results.

<?php

if (strpos("abc", "a") == false) {
    echo "not found.";
}

This tool gives hints to the above code.