problem filling out form with capital letters

Before adding to your database, convert the string to lowercase. I put the down-to-top example of how to convert PHP and Java Script to lowercase.

In PHP:

$string = "TESTE";
echo strtolower($string); //teste

In Javascript:

var string = "TESTE";
console.log(string.toLowerCase()); // teste

To force the field to always be lowercase, it would look like this:

// Função javascript
function lowerCase(input) {
  input.value = input.value.toLowerCase();
}

Tente digitar letras maiúsculas:
<input type="text" id="txt" onkeyup="return lowerCase(this)" />​

I improved the above example. You need to put this javascript function in a .js file and its input, put onkeyup="return lowerCase(this)" .

You can use strtolower and convert, before sending to the database. strtolower

EDITED

<script type="text/javascript">
// INICIO
function minuscula(a){
v = a.value.toLowerCase();
a.value = v;
}
//FIM
</script>
Como usar
<label>Nome:
<input name="nome" type="text" id="nome" size="50" onkeyup="minuscula(this)" />
</label>

I think the ideal would be to actually fix the "this generates a BUG on my site there in the panel" and start to support the uppercase characters normally.

However, there is mb_strtolower() which allows you to reasonably change the characters, but it is very slow .

In this case things like:

Inkeliz
ÍnkÊlÍz
ĮŇЌẸĹĮŻ

It will result in:

inkeliz
ínkêlíz
įňќẹĺįż

But this is not perfect, because not everything has tiny characters (?) or the function simply ignores, as in the cases:

ᾨ
Ⅻ