Why does the PHP json_encode function convert UTF-8 strings to hexadecimal entities?

Since PHP/5.4.0, there is an option called JSON_UNESCAPED_UNICODE. Check it out:

https://php.net/function.json-encode

Therefore you should try:

json_encode( $text, JSON_UNESCAPED_UNICODE );

JSON_UNESCAPED_UNICODE is available on PHP Version 5.4 or later.
The following code is for Version 5.3.

UPDATED

• html_entity_decode is a bit more efficient than pack + mb_convert_encoding.
• (*SKIP)(*FAIL) skips backslashes itself and specified characters by JSON_HEX_* flags.

function raw_json_encode($input, $flags = 0) {
    $fails = implode('|', array_filter(array(
        '\\\\',
        $flags & JSON_HEX_TAG ? 'u003[CE]' : '',
        $flags & JSON_HEX_AMP ? 'u0026' : '',
        $flags & JSON_HEX_APOS ? 'u0027' : '',
        $flags & JSON_HEX_QUOT ? 'u0022' : '',
    )));
    $pattern = "/\\\\(?:(?:$fails)(*SKIP)(*FAIL)|u([0-9a-fA-F]{4}))/";
    $callback = function ($m) {
        return html_entity_decode("&#x$m[1];", ENT_QUOTES, 'UTF-8');
    };
    return preg_replace_callback($pattern, $callback, json_encode($input, $flags));
}

You like to set charset and unescaped unicode

 header('Content-Type: application/json;charset=utf-8');  
 json_encode($data,JSON_UNESCAPED_UNICODE|JSON_PRETTY_PRINT);