Senses
打招呼 to greet
to greet someone with words or gestures; to say hello
打招呼 to notify
to give prior notice; to inform someone informally or let them know about something beforehand
打招呼 to use influence
to use one's influence on another's behalf; to intercede or put in a word for someone
打招呼 to warn
to give a warning or cautionary notice; to alert someone