Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON

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Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON

Archive - Originally posted on "The Horse's Mouth" - 2011-03-22 07:56:56 - Graham Ellis
When you ask your program to "print out an object", you're really looking to convert a data structure into a stream of characters, and it's not really obvious what you're looking to do.

If you try to print out an object in PHP:
print ($result . "\n");
where the $result variable contains an instance of an object, you'll get a message like:

  Catchable fatal error: Object of class station could not be converted 

  to string in /Users/grahamellis/oop/oop5.php on line 108

How do you avoid your program failing like this? There are a number of options.

1. You can run a method on your object that will convert it to a string:
print ($result->describe() . "\n");

2. You can serialize your object using the built in PHP serialise function. This will convert the object to s atring in predetermined format, so that you can unserialize it later ... and you'll get a string which you can save to file, print out, transmit to another computer, etc.

3. You can (from PHP 5.2) serialise the object with json_encode. "JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate." ... [JSON intro ans spec]

Here's an example of JSON encoding:

print (json_encode($result) . "\n");

  {"info":"5268\tBTN\tBN1 3XP\tTQ 309050\t50.8289532182\t-0.141225193\tBrighton

  \t11295080\t11854512\t12853442\t13474555\t13806628\t13741582","cached":1,

  "fields":["5268","BTN","BN1 3XP","TQ 309050","50.8289532182","-0.141225193",

  "Brighton","11295080","11854512","12853442","13474555","13806628","13741582"]}

4. You can also trap your fatal error in PHP. Prior to any code that may fail and need to be caught, set up an error handler:

set_error_handler('cat_chit');

and that will divert errors from the default handler to your function cat_chit. Here's an example function:

  function cat_chit($errno, $errstr, $errfile, $errline) {

    if ( E_RECOVERABLE_ERROR===$errno ) {

      print ("\n\ncaught a normlly fatal error\n");

      print ("Error number: $errno\n");

      print ("Error string: $errstr\n");

      print ("Error file: $errfile\n");

      print ("Error line: $errline\n");

      return true;

    }

    return false;

  }

The true return tells PHP that we're done with handling the error and it may proceed; the false return indicates that the error has not been (fully) handled by your handler and that the default one should also be called.

So .... 4 possible solutions - which is "best"? It's for you to decide the context. Overriding the error handler makes me worry - you're may be just masking a coding issue. Running a method (which you may need to write) on the object to present it in human-readable form is best for a production program. Serializing or json_encoding is best if you want to store the object, send it to another system, or print it out for analysis and debug purposes.

On yesterday's Object Oriented PHP Course, I developed an example during the day that showed many OO related features of PHP ... and some other features too, including the JSON and error_handler examples above. You can see then in full source code context [here].