derick@singlemalt:~ $ php -dxdebug.overload_var_dump=0 -d display_errors=1 -d display_startup_errors=1 -d error_reporting=E_ALL -d xdebug.auto_trace=1 -d xdebug.trace_format=0 -d xdebug.trace_options=0 -d xdebug.collect_params=1 -d xdebug.collect_return=1 -d xdebug.trace_output_dir=/tmp -d xdebug.trace_output_name=trace /tmp/518-ns.php

derick@singlemalt:~ $ cat /tmp/trace.xt

TRACE START [2018-08-14 10:17:03]

0.0002     425552   -> {main}() /tmp/518-ns.php:0

0.0002     425936     -> call_user_func:{/tmp/518-ns.php:8}(class Closure, long, class Closure) /tmp/518-ns.php:8

0.0002     425936       -> A{closure}(long, class Closure) /tmp/518-ns.php:8

0.0002     425936        >=> NULL

0.0002     425936      >=> NULL

0.0003      50032

TRACE END   [2018-08-14 10:17:03]

derick@singlemalt:~ $ cat /tmp/518-ns.php

<?php

namespace A;
$a = function(){
};

call_user_func($a, 1, $a);