String concatenation vs array implode in PHP - php

Question

Having used Java for a long time my standard method for creating long strings piece by piece was to add the elements to an array and then implode the array.
$out[] = 'a';
$out[] = 'b';
echo implode('', $out);
But then with a lot of data.
The (standard PHP) alternative is to use string concatenation.
$out = 'a';
$out .= 'b';
echo $out;
To my surprise there seems to be no speed difference between both methods. When there is significant time difference usually it is the concatenation that seems faster, but not all of the time.
So my question is: are there - apart from style and code readability - any other reasons to choose one approach over the other?

Answer 1

To me, using an array implies that you're going to do something that can't be done with simple string concatenation. Like sorting, checking for uniqueness, etc. If you're not doing anything like that, then string concatenation will be easier to read in a year or two by someone who doesn't know the code. They won't have to wonder whether the array is going to be manipulated before imploded.
That said, I take the imploded array approach when I need to build up a string with commas or " and " between words.

Answer 2

Choose the more readable one. Always. This case, i would pick up the second apporach.
Then optimize it, if it's a bottleneck.

Answer 3

One (subtle) difference is clearly visible when generating a character-seperated string:
<?php
$out[] = 'a';
$out[] = 'b';
echo implode(',', $out);
foreach($out as $o) {
echo $o . ',';
}
?>
The first one will print a,b where the latter will print a,b,. So unless you're using an empty string as a seperator, as you did in your example, it's usually preferred to use implode().

Answer 4

The concatenation-vs-implode holy war aside: No, there is no difference.

Answer 5

Here is a performance test for both approaches:
<?php
const M = 1000000;
// Imploding method
$start = microtime(true);
$arr = [];
for ($i = 0; $i < M; $i++) {
$arr[] = chr(65 + ($i & 0x7));
}
$str1 = implode('', $arr);
$time1 = microtime(true) - $start;
// Concatenation method
$start = microtime(true);
$str2 = '';
for ($i = 0; $i < M; $i++) {
$str2 .= chr(65 + ($i & 0x7));
}
$time2 = microtime(true) - $start;
assert( $str1 == $str2 );
echo "Time 1: $time1\n";
echo "Time 2: $time2\n";
Output with PHP 7.1.33 on MacOS:
Time 1: 0.121246
Time 2: 0.059228
Output with PHP 8.0.5 on a (slow) Debian:
Time 1: 0.88076496124268
Time 2: 0.8109610080719
So, concatenation method works faster.

Answer 6

it depends on what you want to do with the string / array and how you create it
if you start with an array and need to sort it / manipulate certain elements, then i suggest implode
other than that i usually use concatenation