How can we validate user input in PHP without using built-in functions?

To validate user input in PHP without using built-in functions, you can implement custom validation logic in your PHP code. One common approach is to use regular expressions to validate input based on specific requirements such as format, length, or character restrictions.

Here is an example of how you can validate user input in PHP without using built-in functions:

```html

<?php
$input = $_POST['user_input'];

// Custom validation logic
if (preg_match('/^[a-zA-Z0-9]*$/', $input)) {
echo '

User input is valid

';
} else {
echo '

User input is not valid. Please enter alphanumeric characters only.

';
}
?>

```

In the code above, we use a regular expression `'/^[a-zA-Z0-9]*$/'` to ensure that the user input contains only alphanumeric characters. You can modify the regular expression pattern based on your specific validation requirements.

By implementing custom validation logic in PHP, you can effectively validate user input without relying on built-in functions. It allows you to have more control over the validation process and tailor it to your specific needs.