Spaces When Concatenating Strings in PHP

Let two strings be stored in variables, and when concatenating them, we want to insert a space between them. This is done as follows:

<?php
	$str1 = 'abc';
	$str2 = 'def';
	echo $str1 . ' ' . $str2; // outputs 'abc def'
?>

Let there be only one variable:

<?php
	$str = 'abc';
	echo $str . ' ' . 'def'; // outputs 'abc def'
?>

In this case, there is no point in выделять пробел, as a separate string - we can insert it as part of the second term:

<?php
	$str = 'abc';
	echo $str . ' def'; // outputs 'abc def'
?>

Create a variable with the text 'hello' and a variable with the text 'world'. Using these variables and the string concatenation operation, output the string 'hello world' to the screen.