How to Append and Concatenate Strings in PHP

PHP has two string operators. The concatenation operator (.) joins two strings into a new one, and the concatenating assignment operator (.=) appends a string to the end of an existing variable. Both work in any PHP code, including WordPress themes and plugins.

The concatenation operator

The dot returns a new string made from its left and right arguments. The original variable doesn’t change: after this code, $a is still "Hello " and $b holds "Hello World!".

$a = "Hello ";
$b = $a . "World!"; // now $b contains "Hello World!"

The space at the end of "Hello " is what separates the two words. The dot doesn’t add any spaces for you.

The concatenating assignment operator

.= appends the argument on the right to the variable on the left. $a .= "World!"; is a shorter way of writing $a = $a . "World!";.

$a = "Hello ";
$a .= "World!"; // now $a contains "Hello World!"

A WordPress example

.= is handy when you build a string in steps, such as a file URL. get_template_directory_uri() returns the URL of the active theme’s folder (the parent theme if you use a child theme), and the next line appends the path of an image inside it.

<?php
$i_url = get_template_directory_uri();
$i_url .=  "/assets/img/banner-dummy.jpg";
echo $i_url;
?>

echo then prints the full URL of assets/img/banner-dummy.jpg inside the theme folder. For a file that lives in a child theme, use get_stylesheet_directory_uri() instead; child and parent theme paths compares all four theme path functions. When you print the URL inside an HTML attribute, wrap it in esc_url(), one of the WordPress escaping functions.

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