Get Null value in Json

Hello. I'm trying to create script which mysql query is using more than 1 INNER JOIN (in script pasted below i'm using only one INNER JOIN but it doesn't work too). Sql query generated by this script is correct (checked by: echo $sQuery; and pasted to phpmyadmin). But when i try to display my json - echo json_encode( $output ); - in column which should be value from geo_city.id i get a NULL value. All other fields are returned correctly from the database. Thanks for help. Greetings

My php code:

<?php
	
$aColumns = array(
'engine',
'browser',
'platform',
'version',
'grade',
'geo_city.id' 
);

$sIndexColumn = "ajax.id";	
	
$sTable = "ajax";
		
$sJoin = 'INNER JOIN geo_city ON ajax.id = geo_city.id';
	
/* Database connection information */
	$gaSql['user']       = "";
	$gaSql['password']   = "R";
	$gaSql['db']         = "";
	$gaSql['server']     = "";
	

	/* 
	 * Local functions
	 */
	function fatal_error ( $sErrorMessage = '' )
	{
		header( $_SERVER['SERVER_PROTOCOL'] .' 500 Internal Server Error' );
		die( $sErrorMessage );
	}

	
	/* 
	 * MySQL connection
	 */
	if ( ! $gaSql['link'] = mysql_pconnect( $gaSql['server'], $gaSql['user'], $gaSql['password']  ) )
	{
		fatal_error( 'Could not open connection to server' );
	}

	if ( ! mysql_select_db( $gaSql['db'], $gaSql['link'] ) )
	{
		fatal_error( 'Could not select database ' );
	}

	/* 
	 * Paging
	 */
	$sLimit = "";
	if ( isset( $_GET['iDisplayStart'] ) && $_GET['iDisplayLength'] != '-1' )
	{
		$sLimit = "LIMIT ".intval( $_GET['iDisplayStart'] ).", ".
			intval( $_GET['iDisplayLength'] );
	}
	
	
	/*
	 * Ordering
	 */
	$sOrder = "";
	if ( isset( $_GET['iSortCol_0'] ) )
	{
		$sOrder = "ORDER BY  ";
		for ( $i=0 ; $i<intval( $_GET['iSortingCols'] ) ; $i++ )
		{
			if ( $_GET[ 'bSortable_'.intval($_GET['iSortCol_'.$i]) ] == "true" )
			{
				$sOrder .= "".$aColumns[ intval( $_GET['iSortCol_'.$i] ) ]." ".
					($_GET['sSortDir_'.$i]==='asc' ? 'asc' : 'desc') .", ";
			}
		}
		
		$sOrder = substr_replace( $sOrder, "", -2 );
		if ( $sOrder == "ORDER BY" )
		{
			$sOrder = "";
		}
	}
	
	
	/* 
	 * Filtering
	 * NOTE this does not match the built-in DataTables filtering which does it
	 * word by word on any field. It's possible to do here, but concerned about efficiency
	 * on very large tables, and MySQL's regex functionality is very limited
	 */
	$sWhere = "";
	if ( isset($_GET['sSearch']) && $_GET['sSearch'] != "" )
	{
		$sWhere = "WHERE (";
		for ( $i=0 ; $i<count($aColumns) ; $i++ )
		{
			if ( isset($_GET['bSearchable_'.$i]) && $_GET['bSearchable_'.$i] == "true" )
			{
				$sWhere .= "".$aColumns[$i]." LIKE '%".mysql_real_escape_string( $_GET['sSearch'] )."%' OR ";
			}
		}
		$sWhere = substr_replace( $sWhere, "", -3 );
		$sWhere .= ')';
	}
	
	/* Individual column filtering */
	for ( $i=0 ; $i<count($aColumns) ; $i++ )
	{
		if ( isset($_GET['bSearchable_'.$i]) && $_GET['bSearchable_'.$i] == "true" && $_GET['sSearch_'.$i] != '' )
		{
			if ( $sWhere == "" )
			{
				$sWhere = "WHERE ";
			}
			else
			{
				$sWhere .= " AND ";
			}
			$sWhere .= "".$aColumns[$i]." LIKE '%".mysql_real_escape_string($_GET['sSearch_'.$i])."%' ";
		}
	}
	
	
	/*
	 * SQL queries
	 * Get data to display
	 */
	$sQuery = "
		SELECT SQL_CALC_FOUND_ROWS ".str_replace(" , ", " ", implode(", ", $aColumns))."
		FROM   $sTable
		$sJoin
		$sWhere
		$sOrder
		$sLimit
		";
		//echo $sQuery;
	$rResult = mysql_query( $sQuery, $gaSql['link'] ) or fatal_error( 'MySQL Error: ' . mysql_errno() );
	
	/* Data set length after filtering */
	$sQuery = "
		SELECT FOUND_ROWS()
	";
	$rResultFilterTotal = mysql_query( $sQuery, $gaSql['link'] ) or fatal_error( 'MySQL Error: ' . mysql_errno() );
	$aResultFilterTotal = mysql_fetch_array($rResultFilterTotal);
	$iFilteredTotal = $aResultFilterTotal[0];
	
	/* Total data set length */
	$sQuery = "
		SELECT COUNT(".$sIndexColumn.")
		FROM   $sTable
                $sJoin
	";
	$rResultTotal = mysql_query( $sQuery, $gaSql['link'] ) or fatal_error( 'MySQL Error: ' . mysql_errno() );
	$aResultTotal = mysql_fetch_array($rResultTotal);
	$iTotal = $aResultTotal[0];
	
	
	/*
	 * Output
	 */
	$output = array(
		"sEcho" => intval($_GET['sEcho']),
		"iTotalRecords" => $iTotal,
		"iTotalDisplayRecords" => $iFilteredTotal,
		"aaData" => array()
	);


	while ( $aRow = mysql_fetch_array( $rResult ) )
	{
		$row = array();
		for ( $i=0 ; $i<count($aColumns) ; $i++ )
		{
			if ( $aColumns[$i] == "version" )
			{
				/* Special output formatting for 'version' column */
				$row[] = ($aRow[ $aColumns[$i] ]=="0") ? '-' : $aRow[ $aColumns[$i] ];
			}
			else if ( $aColumns[$i] != ' ' )
			{
				/* General output */
				$row[] = $aRow[ $aColumns[$i] ];
			}
		}
		$output['aaData'][] = $row;
	}
	
	echo json_encode( $output );
	
?>

Fragment of my Json output:

 {"sEcho":0,"iTotalRecords":"57","iTotalDisplayRecords":"57","aaData":[["Trident","Internet Explorer 4.0","Win 95+","4","X",null],["Trident","Internet Explorer 5.0","Win 95+","5","C",null]]}