function sqlStatement($session, $query)
{
    if (!mysql_query($query, $session->fDbInfo)) {
        echo '<p>+++ SQL-Fehler: ' . htmlentities(mySql_error()) . " <br/>{$query}</p>";
    }
}
function sqlStatement(&$session, $query)
{
    $session->trace(TC_Db1, 'sqlStatement: ' . $query);
    dbInit($session);
    if (!mysql_query($query, $session->fDbInfo)) {
        echo '<p>+++ SQL-Fehler: ' . htmlentities(mySql_error()) . '<br/>' . htmlentities($query) . '</p>';
    }
}
<?php

include_once 'userroles.php';
session_start();
checkLogin('admin');
$id = $_POST['id'];
$id = mysql_real_escape_string($id);
$reviewee = $_POST['reviewee'];
$reviewee = mysql_real_escape_string($reviewee);
$testQuery = "SELECT * FROM peerreview WHERE reviewer = '{$id}' AND reviewee = '{$reviewee}'";
$testResult = mysql_query($testQuery) or die($result_string = "Query failed:" . mysql_error());
if (mysql_num_rows($testResult) == 1) {
    $query = "DELETE FROM peerreview  WHERE reviewer = '{$id}' AND reviewee = '{$reviewee}' LIMIT 1";
    $result = mysql_query($query);
}
// Could probably use some error handling or response
echo mySql_error();