You shouldn't search through that array, but use database capabilities for this
Suppose you're passing username through GET form:
if (isset($_GET['search'])) {
$search = mysql_real_escape_string($_GET['search']);
$sql = "SELECT * FROM users WHERE username = '$search'";
$res = mysql_query($sql) or trigger_error(mysql_error().$sql);
$row = mysql_fetch_assoc($res);
if ($row){
print_r($row); //do whatever you want with found info
}
}