The other answer and comment helped me to understand the problem:

  

There may be str_replace() in that namespace, which is not the native language, and therefore \ is specified to make the correct reference. - @ Havenard

However I believe that an example is needed for a better understanding of why.

The reason for using \ in front of native functions such as \str_replace(...) is because there is usually a function with the same name inside (not native) within the namespace, for example:

src / Foo / utils.php:

<?php
namespace Foo;

function str_replace($a, $b, $str)
{
    //Isto chama a função nativa str_replace
    return strtolower(\str_replace($a, $b, $str));
}

function foo($a, $b, $c)
{
    //Isto chama Foo\str_replace
    return str_replace($a, $b, $c);
}

Using:

<?php
include 'src/Foo/utils.php';

Foo\foo('a', 'b', 'abcabcabc');

It was necessary to add \ to this line return strtolower(\str_replace($a, $b, $str)); because if it did not it would be recursively / "infinitely" calling Foo\str_replace , since we are within Foo . If we do

At the moment we use \str_replace we call the function that is outside the namespace , which in this case is a native function.

But if we do this:

<?php
namespace Foo;

function str_replace($a, $b, $str)
{
    //Isto chama Foo\str_replace novamente
    return strtolower(str_replace($a, $b, $str));
}

function foo($a, $b, $c)
{
    //Isto chama Foo\str_replace
    return str_replace($a, $b, $c);
}

The following error will occur:

  

Fatal error: Allowed memory size of 134217728 bytes in Z: \ web \ project \ src \ Foo \ utils.php on line 6

Conclusion

• We should use \ for native functions when declaring another function with the same name within the namespace.

  <?php
  namespace Foo\Bar {
      function str_replace($a, $b, $c) {
           //...Algo aqui
      }
  
      //Aqui usamos Foo\Bar\str_replace, pois é relativo ao namespace
      echo str_replace('a', 'b', 'abc');
  
      //Aqui chamamos a função nativa
      echo \str_replace('a', 'b', 'abc');
  }
  

• There is no need to use \ if not there is a function with the same name as a native function

  <?php
  namespace Foo\Bar {
      //Aqui o PHP chama a função nativa pois não existe a função Foo\Bar\str_replace
      echo str_replace('a', 'b', 'abc');
  }
  

• Non-native functions declare outside namespace should always use \ up front if used within a namespace other than native functions .

  Vendor file / Foo / Bar / Baz.php:

  <?php
  namespace Foo\Bar {
      class Baz {
          public function output()
          {
             echo \test(); //Mostra 'Olá mundo!'
             echo test(); //Causa erro
          }
      }
  }
  

  index.php:

  <?php
  function test() {
      echo 'Olá mundo!';
  }
  
  include 'vendor/Foo/Bar/Baz.php';
  
  Foo\Bar\Baz::output();
  

It is not required but necessary to avoid name conflict or when you need to directly access names in the global scope.

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