Next, we’ve used the CURLOPT_HTTPHEADER option to set the Content-Type header to application/json to inform the API server that we’re sending JSON data.

Post JSON Data Using cURL in PHP

<?php
$url = '{POST_REST_ENDPOINT}';
 
$curl = curl_init();
 
$fields = array(
    'field_name_1' => 'Value 1',
    'field_name_2' => 'Value 2',
    'field_name_3' => 'Value 3'
);
 
$json_string = json_encode($fields);
 
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_POST, TRUE);
curl_setopt($curl, CURLOPT_POSTFIELDS, $json_string);

//If you don't set the Content-Type header, it will use application/x-www-form-urlencoded as the default value.
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type:application/json'));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true );
 
$data = curl_exec($curl);
 
curl_close($curl);