My knowledge of Flex is limited, so please forgive me, but doesn't PHP send information to Flex via XML, rather than just the result variable?
<?php
public function searchItems( $search )
{
$search = mysql_real_escape_string( $search );
$result = mysql_query( "SELECT * FROM tbl_items WHERE item_name OR item_tags LIKE '%$search%'" );
echo '<items>';
while ( $row = mysql_fetch_array( $result ) )
{
echo '<row>';
echo ' <columnnumber1>' . $row[ 0 ] . '</columnnumber1>';
echo ' <columnnumber2>' . $row[ 1 ] . '</columnnumber2>';
echo '</row>';
}
echo '</items>';
}
?>
<mx:DataGrid x="0" y="0" styleName="usermapgrid" height="100%" width="100%" id="datagrid" dataProvider="{amfcall.searchItems(searchstring).lastResult.items.row}" >
<mx:columns>
<mx:DataGridColumn headerText="Column 1" dataField="columnnumber1" visible="true"/>
<mx:DataGridColumn headerText="Column 2" dataField="columnnumber2" visible="true"/>
</mx:columns>
</mx:DataGrid>