Creating a string from a Php array and writing it to a text file

I have an array like this:

Array
(
    [0] => firstname1,
    [1] => lastname1,
    [2] => firstname2,
    [3] => lastname2,
    [4] => firstname3,
    [5] => lastname3
)

Now I want to create a text file containing this content:

firstname1|lastname1#firstname2|lastname2#firstname3|lastname3#

How can I do that?

You can iterate through the array in pairs, contcatenating with your chosen characters, to build a string, and then write it to a file with file_put_contents.

<?php
$names = array(
    'firstname1',
    'lastname1',
    'firstname2',
    'lastname2',
    'firstname3',
    'lastname3'
);

for(
    $i = 0, $n = count($names), $str = '';
    $i < $n;
    $i += 2
)
{
    $str .= $names[$i] . '|' . $names[$i+1] . '#';
}
file_put_contents('/tmp/names.txt', $str);

Or to build the string we could chunk the original array into pairs:

$str = '';
foreach(array_chunk($names, 2) as list($first, $second))
    $str .= $first . '|' . $second . '#';