Insert data if a date doesn't exist in a row (duplicate date)

[curb]

I'm trying to insert data with a date that doesn't currently exist in my database. Some parents are getting the same time and date on their appointment as other parents. How would I have the data insert a random date in a maximum of 7 days?

Example:
It'll generate a date with an existing date.
Dec 4th 2007 11pm
Dec 4th 2007 9pm - problem
Dec 4th 2007 9pm - problem
Dec 4th 2007 9pm - problem
Dec 4th 2007 9pm
Dec 4th 2007 8pm
Dec 4th 2007 7pm
Dec 4th 2007 3am - problem
Dec 4th 2007 3am
Dec 4th 2007 2am - problem
Dec 4th 2007 2am

I currently have this to check if it'll run into duplicate dates but I don't know what to do to keep searching for available dates if duplicate exist.

  $dateformat = "M jS Y ga";

  //pull all the dates from now and up
  $datecheck = mysql_query("SELECT * FROM `" . $db_ext . "_item` WHERE '" . time() . "'<=i_time ORDER BY i_time DESC");
  while ($row = mysql_fetch_array($datecheck)) {
  	  //pull all the dates and see if the new generated dates exist with this one
      $dates .= "" . date($dateformat, $row[i_time]) . "";
  }

  $appointment = mysql_query("SELECT * FROM `" . $db_ext . "_item` ORDER BY i_time DESC");
  echo "<table>";
  while ($row = mysql_fetch_array($appointment)) {
      $rand_day_1 = rand(0, 7);
      $rand_day_2 = "+$rand_day_1 day";
      $rand_hour_1 = rand(0, 14);
      $rand_time_2 = "+$rand_hour_1 hour";
      //unixtime
      $time_current = strtotime("$rand_day_2 $rand_time_2");
      //readable time
      $newdate = "" . date($dateformat, $time_current) . "";

  $finddate = strpos($dates, $newdate);
  if ($finddate === false) {
      //date is available, use this date
      $datesearch = "Available, now use this: $newdate";
  } else {
      //date already exist
      //#######################
      // Since the date isn't available, how would I keep generating numbers until one is available?
      //#######################
      $datesearch = "Not Available, generate until one is: $newdate";
  }

  echo "<tr><td>$datesearch</td></tr>";
  }
  echo "</table>";

[tomhath]

There's no reason to read all the existing appointments into php variables.

All you need to do is create $newdate, then see if it exists in your table by executing a SELECT with it.

But a much better way would be to create $newdate and try to insert it. Make sure you have a UNIQUE KEY on the table so the insert will fail if the date already exists. Check for an error and try again with another date when it happens.

Why is the second way better? Otherwise you would need to wrap the check for an existing appointment and the insert in a transaction to ensure you wouldn't get duplicates.