Is String Numeric

  if (filter_var($MyString, FILTER_VALIDATE_INT) !== False)
  {
    //String is an int
  }
   
    
  if (is_numeric($MyString))
  {
    //String is numeric
  }

Is String A Date Time

  if(strtotime($MyDateTimeString))    //Returns timestamp on success, false otherwise
    //String is in date / time format

Is String Within An Array Of Strings

$MyArrayOfStrings = array ('abcd', 'efgh', 'ijkl' );
if (in_array($MyStringToCheck, $MyArrayOfStrings))
{

}

Check Entered Web Address

This isn’t right – the first test rejects doamins with a hyphen in them

  $domain_name = trim(@mysql_real_escape_string($domain_name));

  if (!eregi("^[_a-zA-Z0-9\-\.]*$", $domain_name))
  {
    $message = "Invalid character(s) in the Domain Name";
  }
  elseif (!eregi("^[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)*(\.[a-z]{2,3})$", $domain_name))
  {
    $message = "Invalid domain name format";
  }

Verify Email Address

A thorough version from this article here

//*****************************************
//*****************************************
//********** CHECK EMAIL ADDRESS **********
//*****************************************
//*****************************************
function check_email_address($email)
{
  //First, we check that there's one @ symbol, and that the lengths are right.
  if (!ereg("^[^@]{1,64}@[^@]{1,255}$", $email))
	{
    //Email invalid because wrong number of characters in one section or wrong number of @ symbols.
    return false;
  }
	
  //Split it into sections to make life easier
  $email_array = explode("@", $email);
  $local_array = explode(".", $email_array[0]);
  for ($i = 0; $i < sizeof($local_array); $i++)
	{
    if (!ereg("^(([A-Za-z0-9!#$%&'*+/=?^_`{|}~-][A-Za-z0-9!#$%&'*+/=?^_`{|}~\.-]{0,63})|(\"[^(\\|\")]{0,62}\"))$", $local_array[$i]))
		{
      return false;
    }
  }
	
  //Check if domain is IP. If not, it should be valid domain name
  if (!ereg("^\[?[0-9\.]+\]?$", $email_array[1]))
	{
    $domain_array = explode(".", $email_array[1]);
    if (sizeof($domain_array) < 2)
		{
      return false; // Not enough parts to domain
    }
		
    for ($i = 0; $i < sizeof($domain_array); $i++)
		{
      if (!ereg("^(([A-Za-z0-9][A-Za-z0-9-]{0,61}[A-Za-z0-9])|([A-Za-z0-9]+))$", $domain_array[$i]))
			{
        return false;
      }
    }
  }
	
  return true;
}