gistfile1.php

<?php
function enhanceins_zip_code_check( $result, $value, $form, $field ) {
	global $wpdb;
	
	if ( !$result[ 'is_valid' ] ) {
		return $result;
	}
	
	if ( !is_array( $value ) ) {
		$value = explode( ',', $value );
	}
	
	foreach ( $value as $val ) {
		$zipcode = '';
		
		if ( isset( $_POST[ 'zipcode_' . $val ] ) ) {
			$zipcode = $_POST[ 'zipcode_' . $val ];
		}
		
		//make sure we have something to work with
		if ( empty( $zipcode ) ) {
			$result[ 'is_valid' ] = false;
			$result[ 'message' ] = 'Please provide a zip code that you want.';
		}
		//make sure it's a valid value as well.
		elseif ( strlen( $zipcode ) > 5 || ! is_numeric( $zipcode ) ) {
			$result[ 'is_valid' ] = false;
			$result[ 'message' ] = 'Please provide a valid zip code.';
		}
		//if we're good so far, check in the database
		else {
			$sql = "SELECT zip FROM " . $wpdb->prefix . "zipcodes WHERE zip = %s LIMIT 1";
			$exists = $wpdb->get_var( $wpdb->prepare( $sql, $zipcode ) );
	
			if ( !empty( $exists ) ) {
				$result[ 'is_valid' ] = false;
				$result[ 'message' ] = 'Please provide an existing zip code';
			}
		}
	}
	
	return $result;
}
add_filter( 'gform_field_validation_3_1', 'enhanceins_zip_code_check', 10, 4);