I want to get predefined user input.
echo 'Enter value: ';
# here write some 'default value'
$value = fgets(STDIN);
So then user get prompt, he can edit input.
Enter value: default value # Here we can backslash and write new one.
How to achieve?
That is not the default/commen way of handling this stuff in shell scripts. In almost any programm you'll find something like this:
Enter your value: [Default Value]
$default = "Default Value";
echo "Enter your value: [", $default, "]";
$result = fgets(STDIN);
if (empty($result)) $result = $default;
Where as the default value gets set when the STDIN of that get was empty.
I'm pretty surr that the thing you want to achive isn't possible at all.