For a block of code if you want to profile it to check how long each statement takes to execute just use the PEAR Benchmark module.

require 'Benchmark/Timer.php';
$timer =& new Benchmark_Timer(true);
$timer->start();
// some setup code here
$timer->setMarker('setup');
// some more code executed here
$timer->setMarker('middle');
// even yet still more code here
$timer->setmarker('done');
// and a last bit of code here
$timer->stop();
$timer->display();

Explanation:
Calling setMarker( ) will records the time. The display( ) method prints out a list of markers, the time when they were set, and elapsed time from previous marker:
-------------------------------------------------------------
marker | time index | ex time | perct
-------------------------------------------------------------
Start | 1029433375.42507400 | - | 0.00%
-------------------------------------------------------------
setup | 1029433375.42554800 | 0.00047397613525391 | 29.77%
-------------------------------------------------------------
middle | 1029433375.42568700 |0.00013899803161621 | 8.73%
-------------------------------------------------------------
done | 1029433375.42582000 | 0.00013303756713867 | 8.36%
-------------------------------------------------------------
Stop | 1029433375.42666600 | 0.00084602832794189 | 53.14%
-------------------------------------------------------------
total | - | 0.0015920400619507 | 100.00%
-------------------------------------------------------------

The Benchmark module also have Benchmark_Iterate class, which is used to time many executions of a single function:

The Benchmark_Iterate::get( ) method returns an associative array. The mean element of this array holds the mean execution time for each iteration of the function. The iterations element holds the number of iterations. The execution time of each iteration of the function is stored in an array element with an integer key. For example, the time of the first iteration is in $results[1], and the time of the 37th iteration is in $results[37].

To automatically record the elapsed execution time after every line of PHP code, use the declare construct and the ticks directive:

The ticks directive is used to execute a function on a repeatable basis for a block of code.

The number assigned to ticks is how many statements go by before the functions that are registered using register_tick_function( ) are executed.

In the previous example, we register a single function and have the profile( ) function execute for every statement inside the declare block. If there are two elements in $_SERVER['argv'], profile( ) is executed four times: once for each time through the
foreach loop, and once each time the print strlen($arg) line is executed.

You can also set things up to call two functions every three statements:

register_tick_function('profile');
register_tick_function('backup');
declare (ticks = 3) {
    // code...
}

You can also pass additional parameters into the registered functions, which can be object methods instead of regular functions:

// pass "parameter" into profile( )
register_tick_function('profile', 'parameter');
// call $car->drive( );
$car = new Vehicle;
register_tick_function(array($car, 'drive'));

If you want to execute an object method, pass the object and the name of the method in encapsulated within an array. This lets the register_tick_function( ) know you're referring to an object instead of a function.

Call unregister_tick_function( ) to remove a function from the list of tick functions:

unregister_tick_function('profile');