Assuming that both pg_connect() and pg_close() are called
inside a function, and then pg_connect() is called again
from the main program, the next error will occur:
"Warning: 1 is not a valid PostgreSQL link resource in ..."

examples:

1. NG

function foo() {
  $conn = pg_connect(...);
  ...
  pg_close($conn);
}

foo(); # calls pg_connect() and pg_close()

$conn = pg_connect(...);  # returns invalid connection id

2. OK

$conn = 0; # defines $conn as a global variable

function foo() {
  global $conn;
  $conn = pg_connect(...);
  ...
  pg_close($conn);
}

foo(); # calls pg_connect() and pg_close()

$conn = pg_connect(...); # returns VALID connection id