@Added PHP API for Zend's ticks

@Added PHP API for Zend's ticks
Added PHP API for registering tick functions.  Ticks is a per-instruction
callback in Zend that allows us to implement cooperative multitasking
within PHP.  Example:

In extension code:

php_add_tick_function(my_tick_func);

void my_tick_func(int count) {
    php_printf("[tick %d]", count);
}

In PHP code:
<?php

declare(ticks = 1) {
    print "there should be two ticks here.\n";
}

?>