how to pass a null value to a foreign key field?

Question

Just allow column university_id of table user to allow NULL value so you can save nulls.

CREATE TABLE user
(
   uid INT NOT NULL,
   Name VARCHAR(30) NOT NULL, 
   university_ID INT NULL,    -- <<== this will allow field to accept NULL
   CONSTRAINT user_fk FOREIGN KEY (university_ID)
       REFERENCES university(university_ID)
)

UPDATE 1

based on your comment, you should be inserting NULL and not ''.

insert into user (name,university_id) values ('harjeet', NULL)

UPDATE 2

$university_id = !empty($university_id) ? "'$university_id'" : "NULL";
insert into user (name,university_id) values ('harjeet', $university_id);

As a sidenote, the query is vulnerable with SQL Injection if the value(s) of the variables came from the outside. Please take a look at the article below to learn how to prevent from it. By using PreparedStatements you can get rid of using single quotes around values.

How to prevent SQL injection in PHP?

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