can not get array value of a simple array in php

You want to return an associative array, which you're trying to do by casting the decoded json (which is an object) into an array. But that doesn't work properly, as you might've seen already. However, the second parameter in the json_decode function is to set the decoder to return an associative array instead of an object - which is exactly what you're looking for. An example of it here:

$encoded = json_encode(array("07" => "GRAY","67"=>"BLUE"));

$decoded = json_decode($encoded);             // Returns stdClass-object
$decoded_array = json_decode($encoded, true); // Return an associative array, 
                                              // which is what you're looking for 

You can see the var_dumps() of $decoded and $decoded_array to be respectively

// var_dump($decoded);
object(stdClass)#7 (2) { 
    ["07"]=> string(4) "GRAY" 
    ["67"]=> string(4) "BLUE" 
} 

// var_dump($decoded_array);
array(2) { 
    ["07"]=> string(4) "GRAY" 
    [67]=> string(4) "BLUE" 
}

The solution

So where you define $colour_hash, you simply need to add a second parameter true to the json_decode function, making it like this

$colour_hash_array = json_decode($matches[1], true);

The result of your output should be

log_me($colour_hash_array['07']); //Returns GRAY
log_me($colour_hash_array['67']); //Returns BLUE

References

• http://php.net/manual/en/function.json-decode.php

When you define your array, you're using integers for your keys (I should say I am assuming this, but it's what seems most likely to me).

So this:

$arr = [
    07 => 'GRAY',
    67 => 'BLUE'
];

Results in this:

Array(
    7 => 'GRAY',
    67 => 'BLUE
)

See how the 0 was dropped from the front of the key? An integer doesn't store that zero.

To reference that key, you can use $arr[7] or $arr['7'].

If it is important for you to keep that zero, then use a string to define your key, instead of an integer, and the zero should remain:

So this:

$arr = [
    '07' => 'GRAY',
    '67' => 'BLUE'
];

Results in this:

Array(
    '07' => 'GRAY',
    '67' => 'BLUE
)

Now that the key is a string, you have to reference it is as the exact string, so only $arr['07'] will work now.

Hope that helps!

Sorry my previus stupid answer, I reproduce this behavior, please see code:

define('WP_DEBUG', true);

function log_me($message) {
    if (WP_DEBUG === true) {
        if (is_array($message) || is_object($message)) {
            error_log(print_r($message, true));
        } else {
            error_log($message);
        }
    }
}
//decode json and convert to array
$colour_hash_array = (array)json_decode('{"07":"GREY", "67":"BLUE", "09":"BLACK"}');

print_r($colour_hash_array);   

foreach($colour_hash_array as $key => $color) {
   print 'type:' . gettype($key) . ':' . $key . "\n";
}

log_me($colour_hash_array['07']);//Return GRAY

log_me($colour_hash_array['67']);//undefine offset

log_me($colour_hash_array['09']);//Return BLACK

But if we use get_object_vars for convert object to array, this code work is fine:

define('WP_DEBUG', true);

function log_me($message) {
    if (WP_DEBUG === true) {
        if (is_array($message) || is_object($message)) {
            error_log(print_r($message, true));
        } else {
            error_log($message);
        }
    }
}

//decode json and convert to array
$colour_hash_array = get_object_vars(json_decode('{"07":"GREY", "67":"BLUE", "09":"BLACK"}'));

print_r($colour_hash_array);


foreach($colour_hash_array as $key => $color) {
   print 'type:' . gettype($key) . ':' . $key . "\n";
}

log_me($colour_hash_array['07']);//Return GRAY

log_me($colour_hash_array['67']);//Return BLUE

log_me($colour_hash_array['09']);//Return BLACK