example.php

<?php

class Foo {
  public function x() {
    sleep(1);
  }
}

add_monitoring();

$foo = new Foo();
$foo->x();

ext.c

ZEND_NAMED_FUNCTION(my_custom_handler)
{
    // Never make it into here...
    original_handler(INTERNAL_FUNCTION_PARAM_PASSTHRU);
}

PHP_FUNCTION(add_monitoring)
{
    zend_function *original_function;
    int handler_index;
    zend_class_entry *ce;

    // https://www.phpinternalsbook.com/php7/extensions_design/hooks.html#overwriting-an-internal-function
    ce = zend_hash_str_find_ptr(EG(class_table), "foo", sizeof("foo") - 1);
    original_function = zend_hash_str_find_ptr(&ce->function_table, "x", sizeof("x")-1);
    original_handler = original_function->internal_function.handler;
    original_function->internal_function.handler = my_custom_handler;
}