How to generate access token and implement google text to speech using Curl PHP

Question

Am trying to implement Text to Speech using Google Text to Speech API. i have been following the documentation here. source link

here is the request sample:

curl -H "Authorization: Bearer "$(gcloud auth application-default print-access-token) -H "Content-Type: application/json; charset=utf-8" --data "{
  'input':{
    'text':'I\'ve added the event to your calendar.'
  },
  'voice':{
    'languageCode':'en-gb',
    'name':'en-GB-Standard-A',
    'ssmlGender':'FEMALE'
  },
  'audioConfig':{
    'audioEncoding':'MP3'
  }
}" "https://texttospeech.googleapis.com/v1/text:synthesize"

here is my coding effort so far

<?php
$apikey ="my keys goes here";
$url = "https://texttospeech.googleapis.com/v1/text:synthesize";
$ch = curl_init();
curl_setopt($ch,CURLOPT_URL, $url);
$params_post ="{
  'input':{
    'text':'I have added the event to your calendar.'
  },
  'voice':{
    'languageCode':'en-gb',
    'name':'en-GB-Standard-A',
    'ssmlGender':'FEMALE'
  },
  'audioConfig':{
    'audioEncoding':'MP3'
  }
}";

curl_setopt($ch, CURLOPT_HTTPHEADER, array("Authorization: Bearer $apikey",'Content-Type: application/json; charset=utf-8'));  
curl_setopt($ch,CURLOPT_CUSTOMREQUEST,'POST');
curl_setopt($ch,CURLOPT_POSTFIELDS, $params_post);
curl_setopt($ch,CURLOPT_SSL_VERIFYPEER,0);
curl_setopt($ch,CURLOPT_SSL_VERIFYHOST,0);
curl_setopt($ch,CURLOPT_RETURNTRANSFER, true);
 echo $response = curl_exec($ch);

curl_close($ch);

print_r($response);


?>

Here is my Issue: when I run the code, it throws error requesting for access token. The main problem is that the documentation URL Link above does not show how to get the access token first.

Here is the error it displayed

{ "error": { "code": 401, "message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.", "status": "UNAUTHENTICATED" } } { "error": { "code": 401, "message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.", "status": "UNAUTHENTICATED" } }