用括号避免出现“PHP严格标准”警告？

Question

在以下代码中，为什么额外的括号会避免警告？

  

PHP严格标准：只应通过引用传递变量。

class Family extends Eloquent
{
    public function members()
    {
        return $this->hasMany('App\Person');
    }   
}

class Person extends Eloquent
{
    public function family()
    {
        return $this->belongsTo('App\Family');
    }

    public function school()
    {
        return $this->belongsTo('App\School');
    }
}

class School extends Eloquent
{
    public function students()
    {
        return $this->hasMany('App\Person');
    }
}