"Array to string conversion" error [duplicate]

I have a page in php to edit files, where you write the name in an input and select the extension by a select, at the time of conversion it happens the error "Array to string conversion". (Follow the code for analysis.)

if(isset($_POST['nome'])){

    if(isset($_POST['ext'])){
     $arquivo = "./Arquivos/".$_POST['nome'].".".$_POST['ext'];
     } else{
     $arquivo = "./Arquivos/".$_POST['nome'].".".['extension'];
     }
     $fp = fopen($arquivo,"w");
     $fw = fwrite($fp,$_POST['texto']);  
     fclose($fp);
     include $arquivo; 
  }