Why my query is not working with the php variable

I'm having problems with my mysql query . The value is passed from the php form correctly with $_POST , however when I add the variable to make the comparison the query does not send a response, but if I add the exact value that is in the database I get a response, but I need to use the value which comes from the variable.

This work works

//pegar matricula
  $query_matricula = mysqli_query($conexao,"SELECT matricula FROM usuario where nome = 'Maria'") or die("ERROR" .mysqli_error());
  $row = mysqli_fetch_assoc($query_matricula);
  $numero_matricula = $row['matricula'];
  echo $row['matricula'];

That way it does not work

 //pegar matricula
  $query_matricula = mysqli_query($conexao,"SELECT matricula FROM usuario where nome = '$nome'") or die("ERROR" .mysqli_error());
  $row = mysqli_fetch_assoc($query_matricula);
  $numero_matricula = $row['matricula'];
  echo $row['matricula'];