implode()

PHP's implode() joins the elements of an array into a single string, placing a separator (the glue) between each element. It's the function you reach for whenever you have a list of values and need one string — a comma-separated list, a URL path, a CSV row, or a SQL IN (...) clause. It is the exact inverse of explode(), which splits a string back into an array.

This page covers the syntax, the return value, and the cases that trip people up: the optional glue, associative arrays, empty arrays, and how non-string values are converted.

Syntax

implode(string $separator, array $array): string
implode(array $array): string   // legacy: separator defaults to ""

Return value: a single string containing all array elements joined by $separator, in their existing order.

The function never changes the original array — it returns a new string.

A basic example

Each element is joined with a single space, producing:

Hello World !

Using a different separator

The glue can be any string, not just a single character. Here we join with a hyphen:

Output:

Hello-World-!

A common real-world use is building a comma-separated list for display or for a query:

<?php
$ids = [4, 7, 12, 19];
echo 'WHERE id IN (' . implode(', ', $ids) . ')';
// WHERE id IN (4, 7, 12, 19)
?>

Joining an associative array

implode() ignores the keys entirely and joins only the values, in insertion order:

<?php
$user = ['id' => 1, 'name' => 'Sam', 'role' => 'admin'];
echo implode(' | ', $user);
?>

Output:

1 | Sam | admin

If you need the keys too (for example id=1&name=Sam), use http_build_query() or array_map() over the keys instead — implode() alone cannot see them.

Edge cases and gotchas

Empty array. Joining an empty array always returns an empty string, no matter the separator:

<?php
echo '[' . implode(', ', []) . ']';   // []
?>

Non-string values are coerced to strings. Numbers, booleans, and null are converted using PHP's normal string rules — which can surprise you: true becomes "1", while false and null become "" (an empty string):

<?php
echo implode(', ', [1, 2.5, true, null, 'x']);
// 1, 2.5, 1, , x
?>

If you want null/false filtered out, run the array through array_filter() before joining.

The separator is optional. Calling implode($array) with no glue concatenates the values directly. Passing the separator first is recommended for readability:

<?php
echo implode(['a', 'b', 'c']);   // abc
?>

Before PHP 8.0 the two arguments could be given in either order (implode($array, $glue)). That historical behavior was removed in PHP 8.0 — always pass the separator first.

implode() vs explode()

implode() and explode() are mirror operations: one builds a string from an array, the other rebuilds the array from the string.

<?php
$csv  = implode(',', ['red', 'green', 'blue']); // "red,green,blue"
$back = explode(',', $csv);                      // ['red', 'green', 'blue']
?>

join() is simply an alias of implode() — the two are interchangeable. See join() and explode() for more.

Practice