16-bit md5 compression into 8-bit string algorithm doubts

Generate an 8-bit random string {code...} For ($ g ^ ord ($ a [$ f + 8]) to generate an 8-bit random string

function make_coupon_card() {        $code = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';        $rand = $code[rand(0,25)]            .strtoupper(dechex(date('m')))            .date('d').substr(time(),-5)            .substr(microtime(),2,5)            .sprintf('%02d',rand(0,99));        for(            $a = md5( $rand, true ),            $s = '0123456789ABCDEFGHIJKLMNOPQRSTUV',            $d = '',            $f = 0;            $f < 8;                $g = ord( $a[ $f ] ),            $d .= $s[ ( $g ^ ord( $a[ $f + 8 ] ) ) - $g & 0x1F ],            $f++        );        return $d;    }  

For ($ g ^ ord ($ a [$ f + 8])-$ g & 0x1F bitwise XOR or subtraction itself and operations, the final range is between 0 and 31. How is this determined?

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Generate an 8-bit random string

function make_coupon_card() {        $code = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';        $rand = $code[rand(0,25)]            .strtoupper(dechex(date('m')))            .date('d').substr(time(),-5)            .substr(microtime(),2,5)            .sprintf('%02d',rand(0,99));        for(            $a = md5( $rand, true ),            $s = '0123456789ABCDEFGHIJKLMNOPQRSTUV',            $d = '',            $f = 0;            $f < 8;                $g = ord( $a[ $f ] ),            $d .= $s[ ( $g ^ ord( $a[ $f + 8 ] ) ) - $g & 0x1F ],            $f++        );        return $d;    }  

For ($ g ^ ord ($ a [$ f + 8])-$ g & 0x1F bitwise XOR or subtraction itself and operations, the final range is between 0 and 31. How is this determined?

( $g ^ ord( $a[ $f + 8 ] ) ) - $g & 0x1F
In short, there is a key point: Operators-The priority&High.
So it should be( ( $g ^ ord( $a[ $f + 8 ] ) ) - $g )And0x1FOperations,
While0x1FIt is in decimal format.31The range of the result to be obtained is limited0 - 31.