Remove "backslash n" (newline) in $_POST data

To remove the new line the following code is working good;

<?php 
$text = "Hello \nwelcome to \ngeeksforgeeks"; 
$text = str_replace("\n", "", $text); 
echo $text; 
?>

OUTPUT:

Hello welcome to geeksforgeeks.

But when I get it as input, it is not working:

<form method="post">
    <label>enter string</label>&nbsp
    <input type="text" name="string"></input><br><br>
    <input type="submit" name="enter"></input>
</form>

<?php 
if (isset($_POST['enter'])) {
    $text = $_POST['string'];
    $out = str_replace("\n", "", $text);
    echo $out;
}
?>

INPUT:

Hello \nwelcome to \ngeeksforgeeks

OUTPUT:

Hello \nwelcome to \ngeeksforgeeks

Please tell me the reason and the fix.

In a double-quoted string, \n is an escape sequence that becomes a newline character. You want to match literal \n in the user input, so you should use a single-quoted string, which doesn't process escape sequences.

if(isset($_POST['enter']))
{
    $text=$_POST['string'];
    $out=str_replace('\n',"",$text);
    echo $out;
}

See What is the difference between single-quoted and double-quoted strings in PHP?