not-prefixed.php

<?php
namespace Foo {
    class Bar {
        public function baz() {
            return strlen('Regarding the overhead: Just keep in mind, that a typical application calls built-in functions many thousand times. Even if the difference is small: It exists.');
        }
    }

    $start = microtime(true);
	$foo = new Bar;
	for($x = 0; $x < 1000000; $x++) {
		$foo->baz();
	}
	echo "Time: " . number_format((microtime(true) - $start), 5, "."," ") . " s\n";
}

prefixed.php

<?php
namespace Foo {
    class Bar {
        public function baz() {
            return \strlen('Regarding the overhead: Just keep in mind, that a typical application calls built-in functions many thousand times. Even if the difference is small: It exists.');
        }
    }

    $start = microtime(true);
	$foo = new Bar;
	for($x = 0; $x < 1000000; $x++) {
		$foo->baz();
	}
	echo "Time: " . number_format((microtime(true) - $start), 5, "."," ") . " s\n";
}

And here come inconclusive bogus that vary only because my cpu usage changes from 3% to 4% occasionally as I move my cursor, or some browser tab refreshes itself in the background. Bottomline, no difference there.

$ php not-prefixed.php 
Time: 1.76156 s
$ php prefixed.php 
Time: 1.75679 s
$ php not-prefixed.php 
Time: 1.74390 s
$ php prefixed.php 
Time: 1.77220 s