SQL 语句中文字前的冒号是什么意思?

What does it mean to use ":" before a variable ?

例如这段代码中的:userId:

public function removeUser($userId)
{
 $command = Yii::app()->db->createCommand();
 $command->delete(
 'tbl_project_user_assignment',
 'user_id=:userId AND project_id=:projectId',
 array(':userId'=>$userId,':projectId'=>$this->id));
}

这是Yii框架中的PHP、MySQL代码.