PHP Try Catch Question: php, try, catch

PHP Try Catch Question

Hi,

I'm inserting records into a mysql database through a form in php. Is it possible to catch an error thrown by the msql database and redisplay the form that is being submitted so the entered values are not lost? I know I can check the values with javascript before the form is submitted, but I was wondering if this can be done with php and try catch blocks??
I'm using mysql 5.0.16 and php 5.1.1

Thanks for your time.

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When a mySQL query fails, it would be very near impossible to associate an error message to a specific field in your form. Before your data reaches mySQL, you should validate each field to ensure it has an acceptable value. And you should do this through PHP (or other serverside technology) rather than javascript. (Javascript validation can still be helpful in addition to server-side validation, but shouldn't be relied upon exclusively).
See below for a quick example of validation using PHP:

<?php
//validation
if( isset($_POST['submit']) ) { //only validate if we have submitted
      if( !strlen($_POST['variable1']) > 0 )
       $error['variable1'] = "Variable 1 must be set!";

      if( !strlen($_POST['variable2']) > 0 )
       $error['variable2'] = "Variable 2 must be set!";
      else if( !is_numeric($_POST['variable2']) )
       $error['variable2'] = "Variable 2 must be a number!";
}
if( !isset($error) ) { //if no errors
//then do your database stuff
}

//display form
?>
<form method="post" action="<?= $_SERVER['PHP_SELF'] ?>">
var1: <input type="text" name="variable1" value="<?php if( isset($_POST['variable1']) ) echo $_POST['variable1']; ?>" /> <?php if( isset($error['variable1']) ) echo $error['variable1']; ?><br />

var2: <input type="text" name="variable2" value="<?php if( isset($_POST['variable2']) ) echo $_POST['variable2']; ?>" /> <?php if( isset($error['variable2']) ) echo $error['variable2']; ?><br />
<input type="submit" name="submit" value="Submit" />
</form>

<?php
if( isset($_POST['submit']) )
      if(!isset($error) )
            echo "<b>Form OK!</b>\n";
      else
            echo "<b>Form has " . sizeof($error) . " error(s).\n</b>";
?>
//end code

Just an example. The above checks that variable1 has a value and that variable2 is a number. If either is not the case, then it outputs an error message to the right of the field... You would likely want to standardize your own functions for error checking, which would allow you to rewrite the above more efficiently with standardized error messages.

The point to take home from all this is that you yourself must understand what values are acceptable for the underlying database.

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