title: What is the type of NULL in mysqli bind_param?
body: <p>I am trying to bind params to a INSERT INTO MySQLi prepared statement if that variable exists, otherwise insert null.Then i know </p>
  
  <pre>
  type    variable
  i   corresponding variable has type integer
  d   corresponding variable has type double
  s   corresponding variable has type string
  b   corresponding variable is a blob and will be sent in packets
  </pre>
  
  <p>but when i insert a variable = null,how i define the type of the variable? Or should i define the type is ''?  </p>