<?php
	
	$JsonString = '{
	"cols": [
		{"id":"","label":"Topping","pattern":"","type":"string"},
		{"id":"","label":"Slices","pattern":"","type":"number"}
	],
	"rows": [
		{"c":[{"v":"Mushrooms","f":null},{"v":3,"f":null}]},
		{"c":[{"v":"Onions","f":null},{"v":1,"f":null}]},
		{"c":[{"v":"Olives","f":null},{"v":1,"f":null}]},
		{"c":[{"v":"Zucchini","f":null},{"v":1,"f":null}]},
		{"c":[{"v":"Pepperoni","f":null},{"v":2,"f":null}]}
	]
}';
	$data = json_decode($JsonString);
	
	$value = "Bla Bla Bla";
	
	$data->rows[0]->c[0]->v = $value;
	
	$JsonString = json_encode($data);
	
	print_r($JsonString);