Migrating from PHP 4 to PHP 5.0.x
PHP Manual

Backward Incompatible Changes

Although most existing PHP 4 code should work without changes, you should pay attention to the following backward incompatible changes:

Example #1 strrpos() and strripos() now use the entire string as a needle

<?php
var_dump
(strrpos('ABCDEF','DEF')); //int(3)

var_dump(strrpos('ABCDEF','DAF')); //bool(false)
?>

Example #2 An object with no properties is no longer considered "empty"

<?php
class test { }
$t = new test();

var_dump(empty($t)); // echo bool(false)

if ($t) {
    
// Will be executed
}
?>

Example #3 In some cases classes must be declared before used

<?php

//works with no errors:
$a = new a();
class 
{
}


//throws an error:
$a = new b();

interface 
c{
}
class 
implements {


?>


Migrating from PHP 4 to PHP 5.0.x
PHP Manual

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