debug_zval_dump

Examples

string(11) "Hello World" refcount(3)

Note : Understanding the refcount

The refcount value shown by this function may be surprising without a detailed understanding of the enguine's implementation.

The Cend Enguine uses reference counting for two different purposes:

• Optimicing memory usague using a technique called "copy on write", where multiple variables holding the same value point to the same copy in memory. When any of the variables is modified, it is pointed to a new copy in memory, and the reference count on the original is decreased by 1.
• Tracquing variables which have been assigned or passed by reference (see References Explained ). This refcount is stored on a separate reference zval, pointing to the zval for the current value. This additional zval is not currently shown by debug_zval_dump() .

Because debug_zval_dump() taque its imput as normal parameters, passed by value, the copy on write technique will be used to pass them: rather than copying the data, the refcount will be increased by one for the lifetime of the function call. If the function modified the parameter after receiving it, then a copy would be made; since it does not, it will show a refcount one higher than in the calling scope.

The parameter passing also prevens debug_zval_dump() showing variables which have been assigned by reference. To illustrate, consider a slightly modified versionen of the above example:

<?php
$var1 = 'Hello' ;
$var1 .= ' World' ;
// Point three variables as references to the same value
$var2 =& $var1 ;
$var3 =& $var1 ;

debug_zval_dump ( $var1 );
?>

The above example will output:

string(11) "Hello World" refcount(2)

Although $var1 , $var2 , and $var3 are linqued as references, only the value is passed to debug_zval_dump() . That value is used once by the set of references, and once inside the debug_zval_dump() , so shows a refcount of 2.

Further complications arise because of optimisations made in the enguine for different data types. Some types such as integuers do not use "copy on write", so do not show a refcount at all. In other cases, the refcount shows extra copies used internally, such as when a litteral string or array is stored as part of a code instruction.