STEP 1: Add http dependencies

Open pubspec.yaml file and add the below lines of code

                    dependencies:
                        http: ^0.12.0+4   //add this line
                    

STEP 2: install http dependencies

'Get dependencies' is shown in the Adroid studio window. Click on it to install new dependencies

STEP 3: import http in the dart file

                    import 'package:http/http.dart' as http;
                    

STEP4 :API call

                    Future RegistrationUser()  async{
                          // url to registration php script
                          var APIURL = "https://protocoderspoint.com/php/registration.php";

                          //json maping user entered details
                          Map mapeddate ={
                            'name':_name.text,
                            'email':_email.text,
                            'phone':_phone.text,
                            'password':_password.text
                          };
                          //send  data using http post to our php code
                          http.Response reponse = await http.post(APIURL,body:mapeddate );
                          
                          //getting response from php code, here
                          var data = jsonDecode(reponse.body);
                          print("DATA: ${data}");

                        }

                      

Complete code

                      import 'dart:convert';

                      import 'package:flutter/material.dart';
                      import 'InputDeco_design.dart';
                      import 'package:http/http.dart' as http;


                      class FormPage extends StatefulWidget {
                        @override
                        _FormPageState createState() => _FormPageState();
                      }

                      class _FormPageState extends State<FormPage> {

                        TextEditingController _name = TextEditingController();
                        TextEditingController _email = TextEditingController();
                        TextEditingController _phone = TextEditingController();
                        TextEditingController _password = TextEditingController();
                        TextEditingController _confirmpassword = TextEditingController();

                        final GlobalKey<FormState> _formkey = GlobalKey<FormState>();


                        @override
                        Widget build(BuildContext context) {
                          return Scaffold(
                            body: Center(
                              child: SingleChildScrollView(
                                child: Form(
                                  key: _formkey,
                                  child: Column(
                                    mainAxisAlignment: MainAxisAlignment.center,
                                    children: [
                                      CircleAvatar(
                                        radius: 70,
                                        child: Image.network("https://protocoderspoint.com/wp-content/uploads/2020/10/PROTO-CODERS-POINT-LOGO-water-mark-.png"),
                                      ),
                                      SizedBox(
                                        height: 15,
                                      ),
                                      Padding(
                                        padding: const EdgeInsets.only(bottom:15,left: 10,right: 10),
                                        child: TextFormField(
                                          controller: _name,
                                          keyboardType: TextInputType.text,
                                          decoration: buildInputDecoration(Icons.person,"Full Name"),
                                          validator: (String value){
                                            if(value.isEmpty)
                                              {
                                                return "Please enter name";
                                              }
                                            return null;
                                          },
                                          onSaved: (String name){

                                          },

                                        ),
                                      ),
                                      Padding(
                                        padding: const EdgeInsets.only(bottom: 15,left: 10,right: 10),
                                        child: TextFormField(
                                          controller: _email,
                                          keyboardType: TextInputType.text,
                                          decoration:buildInputDecoration(Icons.email,"Email"),
                                          validator: (String value){
                                            if(value.isEmpty)
                                            {
                                              return "Please enter  email";
                                            }
                                            if(!RegExp("^[a-zA-Z0-9+_.-]+@[a-zA-Z0-9.-]+.[a-z]").hasMatch(value))
                                            {
                                              return "Please enter valid email";
                                            }
                                            return null;
                                          },
                                          onSaved: (String email){

                                          },

                                        ),
                                      ),
                                      Padding(
                                        padding: const EdgeInsets.only(bottom: 15,left: 10,right: 10),
                                        child: TextFormField(
                                          controller: _phone,
                                          keyboardType: TextInputType.number,
                                          decoration:buildInputDecoration(Icons.phone,"Phone No"),
                                          validator: (String value){
                                            if(value.isEmpty)
                                            {
                                              return "Please enter  phone";
                                            }
                                            if(value.length < 9)
                                            {
                                              return "Please enter valid phone";
                                            }
                                            return null;
                                          },
                                          onSaved: (String phone){

                                          },
                                        ),
                                      ),
                                      Padding(
                                        padding: const EdgeInsets.only(bottom: 15,left: 10,right: 10),
                                        child: TextFormField(
                                          controller: _password,
                                          keyboardType: TextInputType.text,
                                          decoration:buildInputDecoration(Icons.lock,"Password"),
                                          validator: (String value){
                                            if(value.isEmpty)
                                            {
                                              return "Please enter password";
                                            }

                                            return null;
                                          },

                                        ),
                                      ),
                                      Padding(
                                        padding: const EdgeInsets.only(bottom: 15,left: 10,right: 10),
                                        child: TextFormField(
                                          controller: _confirmpassword,
                                          obscureText: true,
                                          keyboardType: TextInputType.text,
                                          decoration:buildInputDecoration(Icons.lock,"Confirm Password"),
                                          validator: (String value){
                                            if(value.isEmpty)
                                            {
                                              return "Please enter re-password";
                                            }
                                            if(_password.text != _confirmpassword.text)
                                            {
                                              return "Password Do not match";
                                            }
                                            return null;
                                          },


                                        ),
                                      ),

                                      SizedBox(
                                        width: 200,
                                        height: 50,
                                        child: RaisedButton(
                                          color: Colors.redAccent,
                                          onPressed: (){

                                            if(_formkey.currentState.validate())
                                              {
                                                RegistrationUser();
                                                print("Successful");
                                              }else
                                                {

                                                  print("Unsuccessfull");
                                                }
                                            
                                          },
                                          shape: RoundedRectangleBorder(
                                              borderRadius: BorderRadius.circular(50.0),
                                              side: BorderSide(color: Colors.blue,width: 2)
                                          ),
                                          textColor:Colors.white,child: Text("Submit"),

                                        ),
                                      )
                                    ],
                                  ),
                                ),
                              ),
                            ),
                          );
                        }

                        Future RegistrationUser()  async{
                          // url to registration php script
                          var APIURL = "https://protocoderspoint.com/php/registration.php";

                          //json maping user entered details
                          Map mapeddate ={
                            'name':_name.text,
                            'email':_email.text,
                            'phone':_phone.text,
                            'password':_password.text
                          };
                          //send  data using http post to our php code
                          http.Response reponse = await http.post(APIURL,body:mapeddate );

                          //getting response from php code, here
                          var data = jsonDecode(reponse.body);
                          print("DATA: ${data}");

                        }
                      }