What are common issues when upgrading from PHP 4 to PHP 5.6.x?

One common issue when upgrading from PHP 4 to PHP 5.6.x is the change in handling of object references. In PHP 4, objects were assigned by reference by default, while in PHP 5.6.x, objects are assigned by value unless explicitly specified. To fix this, you can use the "&" symbol to explicitly assign objects by reference. 

// PHP 4 style assignment by reference
$obj1 = &$obj2;
```

```php
// PHP 5.6.x style assignment by reference
$obj1 = $obj2;

Keywords

PHP 4 PHP 5.6.x backward compatibility deprecated functions migration errors

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