Simple and powerful function for date range validation in PHP using date() and eval() (So it is probably not 100% safe for user exposure, though input is sanitized somewhat). Syntax is simple and allows for every possible crazy date rules that might occour.

PHP date validator

<?php

/**
 * Validates a date to a specific rule. Rules has the following format:
 * :[date-component]: ==|<|>|>=|<=|!= [number]
 * 
 * So to validate if the date is the first of the month:
 * :d: == 1
 * 
 * And the first monday of the month:
 * :w: == 1 && :d: <= 7
 * 
 * On fridays and mondays:
 * :d: == 1 || :d: == 5
 * 
 * All variables used in date() can be used as a date-component.
 * 
 * if rule is empty all dates are considered valid.
 * @param type $rule
 * @param type $date
 * @return type 
 */
function dateValid($rule,$date){
    $DATE = (int) $date;
    
    if(!$rule) return true;
    
    /* Make it safe for eval by removing function calls and variable references */
    $rule = preg_replace('/\w+\s*\(.*?\)|$/i','',$rule);

    //replace rule notations
    $f = create_function('$str', 'return "date(\'$str[1]\',\$DATE)";');
    /*$f = function($str){
        return "date('$str[1]',\$DATE)";
    };*/
    
    $rule = preg_replace_callback('/:(\w):/i', $f, $rule);
    
    return @eval("return $rule;");
}

?>