Refactoring PHP Code: Transitioning from join() to implode() for Efficient String Handling

Learn how to refactor legacy PHP code by replacing the deprecated join() function with implode(). This guide covers best practices for efficient string handling and provides examples for modernizing your PHP projects.

You open an old file, spot a join() call, and someone on the team says “that’s deprecated, switch it to implode().” So you do. Then you wonder why the code runs exactly the same afterward. Here’s the honest answer: join() was never deprecated, and it never got slower. It’s a straight alias of implode(). Same function, same behavior, same speed. PHP’s own manual labels it in one line: an alias of implode().

So why touch it at all? Consistency. If half your codebase says join() and the other half says implode(), you’re making every reader pause to check whether they do different things. They don’t. Picking one name is a readability call, not a correctness or performance fix. This tutorial walks through implode() for real work, and reframes the join() question as what it actually is: a style preference you can standardize on.

So Is join() Deprecated? No.

Let’s kill the myth directly. join() is not deprecated in any PHP version. It’s an alias, which means it points at the exact same internal function as implode(). You get identical output, identical edge cases, identical performance. Neither one is “more modern” than the other under the hood.

The only reason teams standardize on implode() is that the name reads a little clearer about what it does, and it’s the primary name in the docs. That’s a fine reason. Just don’t tell yourself you’re fixing a bug or a slowdown, because you’re not.

The Basics of implode()

implode() joins array elements into a single string, with an optional separator between each element. You’ll reach for it when formatting lists or outputting CSV-style data. Here’s the syntax:

Basic Syntax of implode()

<?php
implode( string $separator, array $array ) : string

Two parameters, and the order matters:

• $separator: The string placed between each element. It’s optional and defaults to an empty string, so leave it off and the elements join with nothing between them.
• $array: The array whose elements get joined into a string.

One caveat worth knowing if you maintain old code: PHP used to tolerate the reversed order, implode( $array, $separator ). That legacy order was deprecated in PHP 7.4 and removed in PHP 8.0. On modern PHP the separator comes first, full stop. If you’re upgrading an old project, that’s the real thing to grep for, not join().

Example 1: Basic Usage of implode()

Start simple. Join an array of words into a sentence with a space between each one.

Example:

<?php
$words    = ['PHP', 'is', 'a', 'powerful', 'language'];
$sentence = implode( ' ', $words );
echo $sentence;
?>

Output:

PHP is a powerful language

The array joins on a space, and you get a readable sentence back.

Using implode() with Different Separators

The separator can be any character or string, which is what makes implode() flexible across formatting needs. A couple of variations follow.

Example 2: Using Commas as Separators

Say you want a comma-separated list, the kind you’d use for CSV output:

Example:

<?php
$items = ['apple', 'banana', 'orange', 'grape'];
$list  = implode( ', ', $items );
echo $list;
?>

Output:

apple, banana, orange, grape

Same function, different separator, and now you’ve got a comma-separated list ready for export or display.

Refactoring from join() to implode()

Swapping join() for implode() is a rename and nothing more, because they’re the same function. Do it for consistency across a codebase, not because one is safer or faster. The change is safe precisely because nothing about the behavior moves.

Example 3: Refactoring join() to implode()

Say you have legacy code like this:

Before Refactoring:

<?php
$elements = ['car', 'bike', 'plane'];
$result   = join( ', ', $elements );
echo $result;
?>

You can rename it to implode() and move on:

After Refactoring:

<?php
$elements = ['car', 'bike', 'plane'];
$result   = implode( ', ', $elements );
echo $result;
?>

The output is identical. The only thing that changed is that your team now sees one name everywhere.

Advanced Use Cases for implode()

Beyond simple concatenation, implode() pulls its weight in a few more involved scenarios. Here are two you’ll actually hit.

Example 4: Joining Nested Arrays

With a multi-dimensional array, pair implode() with a loop to flatten each row into a string.

Example:

<?php
$nested_array = [
    ['name' => 'John', 'age' => 30],
    ['name' => 'Jane', 'age' => 25],
    ['name' => 'Doe', 'age' => 40],
];
foreach ( $nested_array as $person ) {
    echo implode( ', ', $person ) . "<br>";
}
?>

Output:

John, 30
Jane, 25
Doe, 40

Each sub-array runs through implode() to produce one comma-separated line per person.

Example 5: Using implode() for URL Query Parameters

You can also build a query string from an associative array, handy when you’re constructing URLs for an API call.

Example:

<?php
$params = [
    'search' => 'php implode',
    'page'   => 1,
    'sort'   => 'relevance'
];
$query_string = implode( '&', array_map(
    function ( $key, $value ) {
        return "$key=$value";
    },
    array_keys( $params ),
    $params
));
echo "https://example.com/search?$query_string";
?>

Output:

https://example.com/search?search=php implode&page=1&sort=relevance

Here implode() stitches the pairs together with an ampersand. One honest caveat: this example doesn’t URL-encode the values, so the space in “php implode” stays raw. In real code, reach for http_build_query() instead, which handles both the joining and the encoding for you. Use the hand-rolled version above only when you know your values are already safe.

Performance: implode() vs Manual Concatenation

Let’s be precise about where the performance story is real and where it isn’t. Switching from join() to implode() changes nothing on speed, since they’re the same function. But implode() versus building a string by hand in a loop? That’s a genuine difference, and it’s the one worth caring about.

A couple of things to keep in mind:

• For large arrays, let implode() do the joining instead of appending to a string with .= inside a loop. It’s cleaner to read and avoids repeated string reallocations.
• In performance-critical paths, profile before you assume. Array size and the work you do per element matter more than the join call itself.

Example 6: Avoiding Manual Concatenation with implode()

Rather than concatenating in a loop and trimming the trailing separator, hand the whole array to implode():

Example:

<?php
$items = ['apple', 'banana', 'orange'];
// Instead of manually concatenating strings:
$result = '';
foreach ( $items as $item ) {
    $result .= $item . ', ';
}
echo rtrim($result, ', ');
// Use implode for a cleaner solution:
echo implode( ', ', $items );
?>

The implode() version is shorter, doesn’t need the rtrim() cleanup, and reads at a glance. That’s the win here, not a swap between two identical function names.

Conclusion

Here’s the whole thing in plain terms. join() is not deprecated, it’s not slower, and it’s an exact alias of implode(). If you standardize on implode(), do it for one reason only: one name across the codebase is easier to read than two names that mean the same thing.

Where implode() genuinely earns its place is the everyday work of turning arrays into strings, from simple sentences to CSV rows to query strings, and replacing hand-rolled concatenation loops. And if you’re modernizing older code, the argument order is the thing that actually breaks: separator first, array second, since PHP 8.0. Get that right and the rest is just choosing a name and sticking with it.