Programming PHP

Stacks
Although not as common in PHP programs as in other programs, one fairly common
data type is the last-in first-out (LIFO) stack. We can create stacks using a pair
of PHP functions, array_push( ) and array_pop( ). The array_push( ) function is identical
to an assignment to $array[]. We use array_push( ) because it accentuates the
fact that we’re working with stacks, and the parallelism with array_pop() makes our
code easier to read. There are also array_shift( ) and array_unshift( ) functions for
treating an array like a queue.
Stacks are particularly useful for maintaining state. Example 5-4 provides a simple
state debugger that allows you to print out a list of which functions have been called
up to this point (i.e., the stack trace).
Example 5-4. State debugger
$call_trace = array( );
function enter_function($name) {
global $call_trace;
array_push($call_trace, $name); // same as $call_trace[] = $name
echo "Entering $name (stack is now: " . join(' -> ', $call_trace) . ')';
}
function exit_function( ) {
echo 'Exiting';
global $call_trace;
array_pop($call_trace); // we ignore array_pop( )'s return value
}
function first( ) {
enter_function('first');
exit_function( );
}
function second( ) {
enter_function('second');
first( );
exit_function( );
}
function third( ) {
enter_function('third');
second( );
first( );
exit_function( );
}
first( );
third( );
138 | Chapter 5: Arrays
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