Simple Quotes - Bugando input / POST

Question

When I insert into the <input> COMMENT ON A POST a single quote. ', he makes the whole row existing comment, if I put two quotes, '', he returns only one ('), if I put 3, it all goes away and if I put '-' he returns 0.

HTML:

<form id="comentario"  method="POST" action="assets/php/post.php">
   <input type="hidden" value="<?php echo $id_string; ?>" id="id-publi" name="id">
    <input id="input-comentario" name='i-comentario' type="text" class="comentario" placeholder="Digite seu comentário">
      <center>
      <button class="btn waves-effect waves-light btn " type="submit" id="publicar-button" style="display: none" >Publicar</button></center>
 </form>

PHP:

elseif(isset($_POST['comentario'])){
    $email=$_COOKIE['login_user'];
    $sqlemail="SELECT nome,sobrenome,id FROM usuarios WHERE email='$email'";
    $result=$conn->query($sqlemail);
    $row=$result->fetch_assoc();
    $usuario=$row['id'];
    $publicacao=$_POST['id'];
    $comentario=$_POST['comentario'];
    $sqlcom="INSERT INTO comentario(comentario,id_usuario,id_public) VALUES('$comentario',".$row['id'].",$publicacao)" or die($conn->error);
    if($conn->query($sqlcom)===true){
        $querl="SELECT * FROM comentario WHERE id_public='$publicacao' and id_usuario='$usuario'";
        $resull=$conn->query($querl) or die($conn->error);
        $rowl=$resull->fetch_array();
        $querp="SELECT * FROM publicacao WHERE id_public='$publicacao'";
        $resulp=$conn->query($querp) or die($conn->error);
        $rowp=$resulp->fetch_array();
        $sqlnoti="INSERT INTO noti(usuario,usuario2,comentario,lido)VALUES('".$rowp['id_usuario']."','".$row['id']."','".$rowl['id_com']."',0)";
        $resultnoti=$conn->query($sqlnoti) or die($conn->error);
        $rownoti=$conn->affected_rows;
        $queryfoto="SELECT * FROM infocomplementar WHERE id_usuario='".$row['id']."'";
            $resultfoto=$conn->query($queryfoto) or die($conn->error);
            $rowfoto=mysqli_fetch_array($resultfoto);   
            $nome=$row['nome'];
            $sobrenome=$row['sobrenome'];
            $diretorioperfil="assets/usuarios/fotoperfil/";  
            $id_publi=$publicacao;   

         include "../../comentarios.php";

    }   

Can someone help me find a solution ?