DB backup script - Call to undefined function error - due to if statement

The error

Fatal error: Call to undefined function backup_tables()

The code:

<?php
$doBackupDB=$_REQUEST["doBackupDB"];
if($doBackupDB=='yes')
{

// create new fresh backup
// dont forget to give it hotname, username, password and db requirements in line below
backup_tables($hostname,$username,$password,$dbname);
/* backup the db OR just a table */
// call db variables from your db
function backup_tables($hostname,$username,$password,$dbname,$tables = '*')
{

$link = mysql_connect($hostname,$username,$password);
mysql_select_db($dbname,$link);

//get all of the tables
if($tables == '*')
{
$tables = array();
$result = mysql_query('SHOW TABLES');
while($row = mysql_fetch_row($result))
{
$tables[] = $row[0];
}
}
else
{
$tables = is_array($tables) ? $tables : explode(',',$tables);
}

//cycle through
foreach($tables as $table)
{
$result = mysql_query('SELECT * FROM '.$table);
$num_fields = mysql_num_fields($result);

$return.= 'DROP TABLE IF EXISTS '.$table.';';
$row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
$return.= "\n\n".$row2[1].";\n\n";

for ($i = 0; $i < $num_fields; $i++)
{
while($row = mysql_fetch_row($result))
{
$return.= 'INSERT INTO '.$table.' VALUES(';
for($j=0; $j<$num_fields; $j++)
{
$row[$j] = addslashes($row[$j]);
$row[$j] = ereg_replace("\n","\\n",$row[$j]);
if (isset($row[$j])) { $return.= '"'.$row[$j].'"' ; } else { $return.= '""'; }
if ($j<($num_fields-1)) { $return.= ','; }
}
$return.= ");\n";
}
}
$return.="\n\n\n";
}
//save file
$handle = fopen('backups/db-backup-'.time().'.sql','w+');
fwrite($handle,$return);
fclose($handle);

}


}
?>

The Confusion/Problem

Now I know it's happening because of this if statement:

$doBackupDB=$_REQUEST["doBackupDB"];
if($doBackupDB=='yes')
{
// backup code 
}

Basically I want to trigger this backup by clicking a button, like this:

backup.php?doBackupDB=yes

If I don't use this if statement, backup is being created on page load, because the script executes automatically.

If i try to control it and use an if statement to check that if button was clicked and the page reloads and the variable was passed to dobackup then script should run --- here the error occurs.

Please advise. For now, I have to empty the backup directory before backup script runs, on page load, to get the fresh backup by this script and to delete old.