(PHP 4 >= 4.2.0, PHP 5)
pg_last_error — Get the last error message string of a connection
$connection
  ] )
   pg_last_error() returns the last error message
   for a given connection. 
  
Error messages may be overwritten by internal PostgreSQL (libpq) function calls. It may not return an appropriate error message if multiple errors occur inside a PostgreSQL module function.
Use pg_result_error(), pg_result_error_field(), pg_result_status() and pg_connection_status() for better error handling.
Note:
This function used to be called pg_errormessage().
connection
       PostgreSQL database connection resource.  When 
       connection is not present, the default connection 
       is used. The default connection is the last connection made by 
       pg_connect() or pg_pconnect().
      
   A string containing the last error message on the 
   given connection, or FALSE on error.
  
Example #1 pg_last_error() example
<?php
  $dbconn = pg_connect("dbname=publisher") or die("Could not connect");
  // Query that fails
  $res = pg_query($dbconn, "select * from doesnotexist");
  
  echo pg_last_error($dbconn);
?>