How not to lose the form fields, after validation with PHP

Question

This question differs from the existing questions: How to persist/fill in form data via PHP? and How to pass a value to next page with PHP because my form has between 50 and 100 fields, and therefore a solution using GET or POST is impractical or even impractical.

Although my production form has a lot of fields, I am using the code below with only 2 fields to simplify:

<form class="form" name="frmCadAcolhido" method="post" action="insert.php">
    <input type="hidden" name="acao" value="inserir" >

            <label class="control-label">Nome</label>
            <input type="text" class="control" name="nome" required>

            <label class="control-label">CPF</label>
            <input type="text" class="control" name="cpf" required maxlength="11">

            <input type="submit" name="btnCadastar" value="Cadastrar">
 </form>

When the user clicks on the button, the script in "Insert.php" is submitted. First, it checks whether the CPF entered by the user already exists in the table through a query to the Mysql database. If the CPF does not exist, an INSERT of the fields is made in the table. But if the CPF already exists, it displays an error message and returns to the form. Follow the php code:

include "Conexao.class.php";

$dados = $_POST;

if($dados['acao'] == 'inserir')
{
    $conn = new Conexao();
    $conn->conectaBase();

    $cpf = $dados['cpf'];

    $sql = mysqli_query($conn->link,"SELECT * FROM acolhidos WHERE cpf = '$cpf' ");

    $existecpf = mysqli_num_rows($sql);

    // Se o número do CPF já existe na tabela ACOLHIDOS, retornar mensagem de erro.

    if ($existecpf != 0){
        echo "<script>alert('ERRO: CPF já existe no banco de dados.');</script>";
        echo "<script>history.go(-1)</script>";
    }

    $sql = "INSERT INTO acolhidos
    (
    nome,
    cpf
    )
    VALUES
    (
    '$dados[nome]',
    '$dados[cpf]',
    )";
    $query = mysqli_query($conn->link, $sql);
    echo mysqli_error($conn->link);
}

$conn->desconecta(); 

However, when the script returns to the html form, the contents of all form fields appear empty.

How could I do so that when returning to the form, the data entered by the user remain filled?

The command echo "<script>history.go(-1)</script>"; back to the form page, but with empty fields.

Answer 1

In PHP, if you already have CPF, direct redirect by php passing the data by get

include "Conexao.class.php";

$dados = $_POST;

if($dados['acao'] == 'inserir')
{
    $conn = new Conexao();
    $conn->conectaBase();

    $cpf = $dados['cpf'];
    $nome = $dados['nome'];

    $sql = mysqli_query($conn->link,"SELECT * FROM acolhidos WHERE cpf = '$cpf' ");

    $existecpf = mysqli_num_rows($sql);

    // Se o número do CPF já existe na tabela ACOLHIDOS, retornar mensagem de erro.

    if ($existecpf != 0){
        //redireciona a página para o html passando os dados por get
        header("location: ./caminho/do_formulario.html?cpf=$cpf&nome=$nome&mensagem=CPF%20já%20existe");
        exit;
    }

    $sql = "INSERT INTO acolhidos
    (
    nome,
    cpf
    )
    VALUES
    (
    '$dados[nome]',
    '$dados[cpf]',
    )";
    $query = mysqli_query($conn->link, $sql);
    echo mysqli_error($conn->link);
}

$conn->desconecta();

And in HTML only show the data:

<form class="form" name="frmCadAcolhido" method="post" action="insert.php">
<input type="hidden" name="acao" value="inserir" >

    <label class="control-label">Nome</label>
    <input type="text" class="control" name="nome" value="<?php echo $GET["nome"]; ?>" required>

    <label class="control-label">CPF</label>
    <input type="text" class="control" name="cpf" value="<?php echo $GET["cpf"]; ?>" required maxlength="11">

    <input type="submit" name="btnCadastar" value="Cadastrar">
</form>
<p>
    <?php echo $GET["mensagem"]; ?>
</p>

Answer 2

You are sending a request via form to the file Insert.php, but the page that has the form is HTML and when sending the information the page is updated and in this case it is like a new page, IE, the form fields will be unfilled. It would be better if you used other means to do what you want.

If you want you can use Jquery according to the example I left below:

<form class="form" id="frmCadAcolhido" name="frmCadAcolhido" method="post">
    <input type="hidden" id="acao" name="acao" value="inserir" />
    <label class="control-label">Nome</label>
    <input type="text" class="control" id="nome" name="nome" required />
    <label class="control-label">CPF</label>
    <input type="text" class="control" id="cpf" name="cpf" required maxlength="11" />
    <input type="button" id="btnCadastrar" name="btnCadastrar" value="Cadastrar" />
    <input type="reset" id="btnLimpar" name="btnLimpar" value="Limpar" />
</form>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script>
    $(function(){
        $('#frmCadAcolhido #btnCadastrar').on('click',enviaForm);
    });
    function enviaForm(){
    var acao = $('#frmCadAcolhido #acao'),
    nome = $('#frmCadAcolhido #nome'),
    cpf = $('#frmCadAcolhido #cpf'),
    btn = $('#frmCadAcolhido #btnCadastrar');
    btn.prop({'disabled':true}); // ajuda a evitar duplo click e várias requisições
    if(acao.val()=='inserir'){
        if(nome.val()==undefined || nome.val()==null || nome.val()==''){
            alert('Favor preencher o campo');
            nome.focus();
            btn.prop({'disabled':false});
            return false;
        }
        else if(cpf.val()==undefined || cpf.val()==null || cpf.val()==''){
            alert('Favor preencher o campo');
            cpf.focus();
            btn.prop({'disabled':false});
            return false;
        }
        else{
            $.post('insert.php',
                {acao : "\'" + acao.val() + "\'",nome: "\'" + nome.val() + "\'",cpf: "\'" + cpf.val() + "\'"},
                function(data){
                    alert(data); //Trate o resultado como quiser
                    btn.prop({'disabled':false});
                    return false; // impede a atualização da página e mantém as informações nos campos
                }
            );
        }   
    }else{ alert('Erro ao enviar form');btn.prop({'disabled':false});return false;}
}
</script>

I switched the input "Submit" to "button" and added "id" to all input’s.