get access token from google oauth2 service account json file using jwt token

Here is a simple code snippet that takes google service account .json file as input and gives you access token with the specified scope.

	<?php
	function base64url_encode($data) {
	return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
	}
	$scope = 'https://www.googleapis.com/auth/drive';
	$json=file_get_contents('service_account.json');
	$key = json_decode($json);
	$time=time();
	$header = ['alg'=>'RS256','typ'=>'JWT'];
	$claimset = ['iss'=>$key->client_email,'scope'=>$scope,'aud'=>$key->token_uri,'exp'=>$time+3600,'iat'=>$time];
	$header_base64 = base64url_encode(json_encode($header));
	$claimset_base64 = base64url_encode(json_encode($claimset));
	openssl_sign($header_base64.".".$claimset_base64,$signature,$key->private_key,"SHA256");
	$signature_base64 = base64url_encode($signature);
	$jwt=implode('.',[$header_base64,$claimset_base64,$signature_base64]);
	//echo $jwt;
	$post=[];
	$post['grant_type'] = 'urn:ietf:params:oauth:grant-type:jwt-bearer';
	$post['assertion']=$jwt;
	$curl = curl_init($key->token_uri);
	curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
	curl_setopt($curl, CURLOPT_POSTFIELDS,$post);
	curl_setopt($curl, CURLOPT_BINARYTRANSFER, true);
	curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, 0);
	curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, 0);
	$response = curl_exec($curl);
	$code = curl_getinfo($curl);
	curl_close($curl);
	$token = json_decode($response);
	$access_token = $token->access_token;
	//echo $access_token;

view raw google_oauth2_service_account_jwt_access_token.php hosted with ❤ by GitHub

Hope it helps somebody who is in need of , or been searching for it.