An extremely simple example of how to create a generic validation function using PHP's filter_input() function and/or callbacks

example.php

<?php

//Define how each POST field is validated
$fieldValidation = [
    'username'    => 'required',
    'first_name'  => 'required',
    'last_name'   => 'required',
    'website'     => 'website',
    'email'       => 'email',
    'pass'        => 'password',
];

//validate each field, redirect and display appropriate error messages as needed
$validFields = [];
foreach($fieldValidation as $field => $validationType) {
    $validFields[$field] = validate_post_field($validationType, $field);
}

functions.php

<?php
/**
 * Validates a POST field when provided a valid type and a field name
 * @see http://php.net/manual/en/function.filter-input.php
 * @see http://php.net/manual/en/filter.filters.validate.php
 *
 * @param string $type              A valid type use for validation
 * @param string $fieldName         The name of the POST field
 * @param array  $filterTypeOptions Optional parameters supplied to the filter method
 * @return bool                     True if the field is valid, false otherwise
 */
function validate_post_field($type, $fieldName, array $filterTypeOptions = [])
{
    $filterType = '';
    $alternativeValidationFunction = null;
    
    //determine how the data will be filtered
    switch($type) {
        case 'email':
            $filterType = FILTER_VALIDATE_EMAIL;
            break;
        case 'bool':
            $filterType = FILTER_VALIDATE_BOOLEAN;
            break;
        case 'website':
            $filterType = FILTER_VALIDATE_URL;
            break;
        case 'string':
            $filterType = FILTER_VALIDATE_STRING;
            break;
        case 'required':
            //If the type is too complex to use one of the native validation types, a function can be used instead
            $alternativeValidationFunction = 'validate_not_empty';
            break;
        case 'password':
            $alternativeValidationFunction = 'validate_strong_password';
            break;
        default:
            throw new InvalidArgumentException("The specified data type [$type] is not supported");
            break;
    }
    if($filterType) {
        return filter_input(INPUT_POST, $fieldName, $filterType, $filterTypeOptions);
    }

    $value = isset($_POST[$fieldName]) ? $_POST[$fieldName] : null;
    return $alternativeValidationFunction($value, $filterTypeOptions);
}

function validate_not_empty($value, array $options = [])
{
    //being lazy here, note that a value of "0" will cause this to fail
    return $value != null;
}

function validate_strong_password($value, array $options = [])
{
    //I'll leave this as an exercise for the user
}