PHP/PostgreSQL database writing issue

I'm having problems writing data from a form to multiple datatables on my PostgreSQL database.

Here is my data model

CREATE TABLE institutions(i_id PK, name text, memberofstaff REFERENCES staff u_id);

CREATE TABLE staff(u_id PK, username text, password text, institution REFERENCES institutions i_id);

So its a 1:1 relationship. These tables have been set up fine. It's the PHP script I'm having difficult with. I'm using CTE-datamodifying or at least trying to but I keep receiving errors on submit.

The PHP:

 $conn = pg_connect('database information filled out in code');

       $result = pg_query("WITH x AS (

       INSERT INTO staff(username, password, institution)

       VALUES('$username', '$password', nextval('institutions_i_id_seq'))

       RETURNING u_id, i_id)

       INSERT INTO institutions (i_id, name, memberofstaff)

       SELECT x.i_id, x.u_id, '$institution'

       FROM x");

       pg_close($conn);

So that's the code and the error I get is:

Warning: pg_query() [function.pg-query]: Query failed: ERROR: relation "institutions_i_id_seq" does not exist LINE 3: VALUES('AberLibrary01', '', nextval('institutions_i_id_se... ^ in DIRECTORY LISTING(I replaced this) on line 22

Anyone got any ideas?