PHP（5.5.9）SWIG：如何将指向boolean的指针作为参数传递？

I am using SWIG (3.0.12) with PHP (5.5.9) to build to the following C++ method (in class arith_uint256):

arith_uint256& SetCompact(uint32_t nCompact, bool *pfNegative = NULL, bool *pfOverflow = NULL);

SWIG writes this to the wrapper PHP:

function SetCompact($nCompact, $pfNegative=null, $pfOverflow=null) {
    $r=arith_uint256_SetCompact($this->_cPtr,$nCompact,$pfNegative,$pfOverflow);
    if (!is_resource($r)) return $r;
    return new arith_uint256($r);
}

It compiles fine, and I can call the method without the optional args pfNegative and pfOverflow. However, I can't figure out how to pass by-ref those boolean parameters (passing null is ok, but can't figure out how to pass pointer to boolean).

Explicit pass-by-reference is deprecated on the calling side in PHP. Adding reference symbol "&" to the args in the SWIG-outputted wrapper class also does not help.

PHP emits:

PHP Fatal error: No matching function for overloaded 'arith_uint256_SetCompact' in Uint256/Uint256.php on line 113

The SWIG type in question is: "SWIGTYPE_p_bool"