pg_result_error does NOT work with prepared statements.

So considering pg_query and pg_query_params don't return results on error, and that this would be an extremely handy thing to have, I just ended up writing my own functions wrapping the pg_send_* ones to work like the aforementionned two <em>should</em>. There's not a whole lot of code to them, the comments are mostly references from the libPQ and PHP doc, to explain the behavior.

<?php

class PostgresConnectionError extends Exception {
    public function __construct($last_error) {
        parent::__construct($message);
    }
}

function pg_send_query_sync($connection, string $query) {
    // This function cannot work if any query is already running for that connection because the results may then get mixed up.
    assert(pg_get_result($connection) === false);

    $dispatch_ok = pg_send_query($connection, $query);

    // The *only* case where nothing can be returned, when the connection fails to dispatch the initial query.
    if (!$dispatch_ok) throw new PostgresConnectionError(pg_​last_​error($connection));

    // From the libPQ doc : "PQgetResult must be called repeatedly until it returns a null pointer, indicating that the command is done."

    // In the case of pg_query/PQExec, which we're trying to emulate:
    // "Note however that the returned PGresult structure describes only the result of the last command executed from the string."
    // "Should one of the commands fail, processing of the string stops with it and the returned PGresult describes the error condition."

    while ($result = pg_get_result($connection)) {
        // Drain all results on the connection and only return the last one.
        if ($last_result) pg_free_result($last_result);
        $last_result = $result;
    }

    assert(is_resource($result) && get_resource_type($result) === "pgsql result");
    return $last_result;
}

function pg_send_query_params_sync($connection, string $query, array $params) {
    // This function cannot work if any query is already running for that connection because the results may then get mixed up.
    assert(pg_get_result($connection) === false);

    $dispatch_ok = pg_send_query_params($connection, $query, $params);

    // The *only* case where nothing can be returned, when the connection fails to dispatch the initial query.
    if (!$dispatch_ok) throw new PostgresConnectionError(pg_​last_​error($connection));

    // From the libPQ doc : "PQgetResult must be called repeatedly until it returns a null pointer, indicating that the command is done."
    
    // In the case of pg_query_params/PQExecParams, which we're trying to emulate:
    // "Unlike PQexec, PQexecParams allows at most one SQL command in the given string. (There can be semicolons in it, but not more than one nonempty command.)"

    while ($result = pg_get_result($connection)) {
        // Drain all results on the connection, although there should only be one.
        if ($last_result) pg_free_result($last_result);
        $last_result = $result;
    }

    assert(is_resource($result) && get_resource_type($result) === "pgsql result");
    return $last_result;
}

You can  use pg_set_error_verbosity() to retrieve SQLSTATUS from pg_last_error().

Because pg_query() returns FALSE if the query fails, you must must use pg_send_query() and pg_get_result() to get the result handle.

PostgreSQL 7.4 introduced a new function called PQresultErrorField() that can be used to get SQLSTATE code from a query, which is far more useful than the error string returned from pg_result_error().
http://www.postgresql.org/docs/7.4/static/libpq-exec.html
http://www.postgresql.org/docs/7.4/static/errcodes-appendix.html

Here's a patch that adds the pg_result_error_field() function to PHP:
http://collapsed.net/patches/php-4.3.5RC3-pg_result_error_field.diff
(To apply the patch cd into your php-4.3.5RC directory and type: patch -p1 </path/to/php-4.3.5RC3-pg_result_error_field.diff)

Example code:
<?php
  if(!($db = pg_connect("user=foo password=bar dbname=foobar")))
    die("pg_connect");

  if(!pg_send_query($db, "SELECT foo FROM bar"))
    die("pg_send_query");

  if(!($result = pg_get_result($db)))
    die("pg_get_result");

  echo(pg_result_error($result) . "<br />\n");

  /* only available if you have patched php */
  if(function_exists("pg_result_error_field"))
  {
    $fieldcode = array(
      "PGSQL_DIAG_SEVERITY",        "PGSQL_DIAG_SQLSTATE",
      "PGSQL_DIAG_MESSAGE_PRIMARY", "PGSQL_DIAG_MESSAGE_DETAIL",
      "PGSQL_DIAG_MESSAGE_HINT",    "PGSQL_DIAG_STATEMENT_POSITION",
      "PGSQL_DIAG_CONTEXT",         "PGSQL_DIAG_SOURCE_FILE",
      "PGSQL_DIAG_SOURCE_LINE",     "PGSQL_DIAG_SOURCE_FUNCTION");

    foreach($fieldcode as $fcode)
    {
      printf("%s: %s<br />\n",
        $fcode,
        pg_result_error_field($result, constant($fcode)));
    }

    pg_free_result($result);
  }
?>

<?php pg_result_error_field($result, PGSQL_DIAG_SQLSTATE); ?>
Returns the SQLSTATE code.