Json_encode - converting an array into a json object - php

Question

I am currently fetching values from mysql table. The values are grabbed from a foreach loop. Then after the loop is done the array is converted into a json object. But I am having difficulties getting the json format in order to use with another api. My loop only display one result when there are two. Also I have a function that returns a hexadecimal value which is returning null when I call it inside the loop. How could I get those values in the format shown below?
header("Content-type: application/json");
//get the course list
$education_query = $db_con->prepare("SELECT a.type, COUNT(1) AS cnt
FROM academy a
GROUP BY a.type");
$education_query->execute();
$data = $education_query->fetchAll();
$output = array();
foreach ($data as $row) {
//$type = ;
$output["valueField"] = $row["type"];
$output["name"] = $row["type"];
$output["color"] = hexcode();
} // foreach ($data as $row) {
echo json_encode(array($output));
Current Result:
[{"valueField":"Upper-Secondary","name":"Upper-Secondary","color":null}]
Desired Result:
[{
valueField: "Post-Secondary",
name : "Post-Secondary",
color: "#40ae18"
},
{
valueField: "Upper-Secondary",
name : "Upper-Secondary",
color: "#aaab4b"
}]
EDIT(adding hexcode function):
function hexcode()
{
$min = hexdec("000000"); // result is 0 and sets the min-value for mt_rand
$max = hexdec("FFFFFF"); // result is 16777215 and sets the max-value for mt_rand
$random = mt_rand($min, $max); // creates a radom number between 0 and 16777215
$random_hex = dechex($random); // transforms the random number into a Hex-Code
// now the test, if the result has 6 chars
if (strlen($random_hex) != "6") // sometimes it returns an only 5, or less, char Hex-Code,
hexcode(); // so the function has to be repeat
else
echo $random_hex; // returns the Hex-Code
}

Answer 1

$output needs to be an array of arrays.
$addToOutput = array();
$addToOutput["valueField"] = $row["type"];
// add other fields
$output[] = $addToOutput;

Answer 2

your loop is displying one results because you're overwriting the array keys every time.
you can change those 3 lines
$output["valueField"] = $row["type"];
$output["name"] = $row["type"];
$output["color"] = hexcode();
to
$output[]["valueField"] = $row["type"];
$output[]["name"] = $row["type"];
$output[]["color"] = hexcode();
can you post the hexcode() function ?
EDIT
no need for all this code, you can use this :
function hexcode(){
return sprintf("#%02X%02X%02X", mt_rand(0, 255), mt_rand(0, 255), mt_rand(0,255));
}