sanitize_numbers() & sanitize_absint()

sanitize_numbers.php

/**
 * Remove any chars except numbers
 * '+1-012-123-45-678' => (string) '101212345678'
 * 'user_15' => (string) '15'
 */
function sanitize_numbers( $string ) {
  $string = is_numeric( $string ) ? (string) $string : $string;

  return is_string( $string ) ? preg_replace( '/\D+/', '', $string ) : '';
}

/**
 * Remove any chars except numbers
 * 'term_15' => (int) 15
 */
function sanitize_absint( $string ) {
  $id = sanitize_numbers( $string );

  return ! empty( $id ) && is_numeric( $id ) ? (int) $id : null;
}