jQuery Grid Plugin - jqGrid

Totally baffled by this.

If I type in the javascript it works, but if I use PHP on the server-side to generate the exact same javascript, then the table still displays but the popup window when you click 'Edit' has no fields.

Here's an example:

colModel :[
{name:'id', index:'id', editable:false,},
{name:'role', index:'role', editable:true, editoptions:{size:20}},
], 

but if I generate that code with something like this:

foreach($fields as $field){
	$column = "";
	$column .= "{";
	$column .= "name:'" . $field . "',";
	$column .= "index:'" . $field . "',";
	if($field == "id"){
		$column .= "editable:'false',";
	} else {
		$column .= "editable:'true',";
		$column .= "editoptions:{size:20}";
	}
	$column .= "},";
	echo $column;
	$columns .= $column;
}

then the table still displays but the popup window when you click 'Edit' has no fields.

Why?