An interesting loophole MYSQL integer and floating point calculation problem

Vulnerability environment

laravel 6.20.15
mysql 5.5
PHP Version 7.3.13
用户属性 brokerage_amount / amount 在MYSQL中类型同为 int(11)

Vulnerability source code

    public function Convert(Request $request)
    {
    
    
        $user = User::find($request->session()->get('id'));
        if (!$user) {
    
    
            $this->error(500, '该用户不存在');
        }
        if ($request->input('brokerage_amount') <= 0) {
    
    
            $this->error(500, '参数错误');
        }
        if ($request->input('brokerage_amount') > $user->brokerage_amount) {
    
    
            $this->error(500, '佣金余额不足');
        }
        $user->brokerage_amount = $user->brokerage_amount - $request->input('brokerage_amount');
        $user->amount = $user->amount + $request->input('brokerage_amount');
        if (!$user->save()) {
    
    
            $this->error(500, '划转失败');
        }
        return response([
            'data' => true,
            'now_brokerage_amount' => $user->brokerage_amount, 
            'now_amount' => $user->amount
        ]);
    }

trigger method

Controllable parameter: brokerage_amount I set it to 0.499999999999999 After submission, $user->brokerage_amount has not decreased. But $user->amount will increase by 1.

effect animation

postscript

Changing the brokerage_amount and amount in the MYSQL type to float can be calculated correctly.
The brokerage_amount is reduced by 0.5 once and the amount is added by 0.5 once

Then change the MYSQL type to float(11) and the brokerage_amount will be reduced by 1 at a time without changing the amount.