<?php

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://api.swep.net/calculateSinglePhase',
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => 'POST',
  CURLOPT_POSTFIELDS =>'{
  "inputs": {
    "calculationMethod": "Design",
    "selectedFluidSide1": {
      "fluidId": 53,
      "isGas": false,
      "inputValue": null
    },
    "selectedFluidSide2": {
      "fluidId": 53,
      "isGas": false,
      "inputValue": null
    },
    "coCurrent": false,
    "selectedHeatExchangers": [
      {
        "heatExchangerInputType": "Parent",
        "id": 1
      }
    ],
    "heatLoad": {
      "amount": 1500,
      "unit": "HeatLoadUnit.Kilowatt"
    },
    "inletTemperatureSide1": {
      "amount": 80,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "inletTemperatureSide2": {
      "amount": 20,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "outletTemperatureSide1": {
      "amount": 40,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "outletTemperatureSide2": {
      "amount": 50,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "flowSide1": null,
    "flowSide2": null,
    "maxPressureDropSide1": {
      "amount": 20000,
      "unit": "PressureUnit.Pascal"
    },
    "maxPressureDropSide2": {
      "amount": 20000,
      "unit": "PressureUnit.Pascal"
    },
    "numberOfPasses": null,
    "numberOfPlates": null,
    "numberOfPlatesStack1": null,
    "numberOfPlatesStack2": null,
    "oversurface": null,
    "portSwitch": true,
    "autoperformance": false
  }
}',
  CURLOPT_HTTPHEADER => array(
    'X-API-KEY: YOUR_API_KEY',
    'Content-Type: application/json'
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;