File_exist shows error in Ubuntu/PHP

I'm a new comer to Linux/Ubuntu so if I make any silly mistake please forgive me.

My code works fine in XP but not in Ubuntu 11.04 (Same PHP/MySQL version)

<?php
  if (!isset($_SESSION['uid'])) {
    header('Location:index.php');
    exit;
  } else {
    if (file_exists("profile_pic/".$_SESSION['uid'].".jpg")) {
      echo '<a href="'.$SitePath.'profile.php?id='.$_SESSION['uid'].'"><img src="'.$SitePath.'profile_pic/'.$_SESSION['uid'].'.jpg" alt="'.$_SESSION['name'].'" title="Me" border="0"/></a>';
    } else {
  echo '<a href="'.$SitePath.'profile.php?id='.$_SESSION['uid'].'"><img src="'.$SitePath.'profile_pic/nopic.jpg" alt="'.$_SESSION['name'].'" title="Me" border="0" /></a>';
  }  
?>

If a file exists with nam开发者_开发知识库e of $_SESSION['uid'].".jpg" then show it, else show an default image.

The error I'm getting

Catchable fatal error: Object of class mysqli_stmt could not be converted to string

You're not showing any database operations, but most likely you've got something like:

$_SESSION['uid'] = mysqli_query("SELECT uid FROM ...");

wherever you're initializing your session data. This is incorrect. The query functions return a statement handle, from which you can fetch the data. In pseudo-code, it'd be:

$result = mysqli_query(...);
$row = $result->fetch();
$_SESSION['uid'] = $row['uid'];

try to print_r($_SESSION['uid']) that will give you a hint what is it, most likely as Marc B said