What is the best way to add two strings together?

String Concatenation with a dot is definitely the fastest one of the three methods. You will always create a new string, whether you like it or not. Most likely the fastest way would be:

$str1 = "Hello";
$str1 .= " World";

Do not put them into double-quotes like $result = "$str1$str2"; as this will generate additional overhead for parsing symbols inside the string.

If you are going to use this just for output with echo, then use the feature of echo that you can pass it multiple parameters, as this will not generate a new string:

$str1 = "Hello";
$str2 = " World";
echo $str1, $str2;

For more information on how PHP treats interpolated strings and string concatenation check out Sarah Goleman's blog.

You are always going to create a new string whe concatenating two or more strings together. This is not necessarily 'bad', but it can have performance implications in certain scenarios (like thousands/millions of concatenations in a tight loop). I am not a PHP guy, so I can't give you any advice on the semantics of the different ways of concatenating strings, but for a single string concatenation (or just a few), just make it readable. You are not going to see a performance hit from a low number of them.

Here's the quick and dirty test code, to understand the performance bottlenecks.

Single concat:

$iterations = 1000000;
$table = 'FOO';
$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = sprintf('DELETE FROM `%s` WHERE `ID` = ?', $table);
}
echo 'single sprintf,',(microtime(true) - $time)."\n";

$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = 'DELETE FROM `' . $table . '` WHERE `ID` = ?';
}
echo 'single concat,',(microtime(true) - $time)."\n";

$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = "DELETE FROM `$table` WHERE `ID` = ?";
}
echo 'single "$str",',(microtime(true) - $time)."\n";

I get these results:

single sprintf,0.66322994232178
single concat,0.18625092506409 <-- winner
single "$str",0.19963216781616

Many concats (10):

$iterations = 1000000;
$table = 'FOO';
$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = sprintf('DELETE FROM `%s`,`%s`,`%s`,`%s`,`%s`,`%s`,`%s`,`%s`,`%s`,`%s` WHERE `ID` = ?', $table, $table, $table, $table, $table, $table, $table, $table, $table, $table);
}
echo 'many sprintf,',(microtime(true) - $time)."\n";

$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = 'DELETE FROM `' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '`,`' . $table . '` WHERE `ID` = ?';
}
echo 'many concat,',(microtime(true) - $time)."\n";

$time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
        $sql = "DELETE FROM `$table`,`$table`,`$table`,`$table`,`$table`,`$table`,`$table`,`$table`,`$table`,`$table` WHERE `ID` = ?";
}
echo 'many "$str",',(microtime(true) - $time)."\n";

Results:

many sprintf,2.0778489112854
many concats,1.535336971283
many "$str",1.0247709751129 <-- winner

As conclusion, it becomes obvious that single concat via dot (.) char is the fastest. And for cases, when you've got many concats, the best performing method is using direct string injection via "injection: $inject" syntax.

Unless its really large amount of text it really doesnt matter.

As the others said, $str1 . $str2 is perfectly OK in most cases, except in (big) loops.
Note that you overlook some solutions:

$output = "$str1$str2";

and for large number of strings, you can put them in an array, and use implode() to get a string out of them.

Oh, and "adding strings" sounds bad, or at least ambiguous. In most languages, we prefer to speak of string concatenation.

What is the best way to add two strings together?

Single concat:

Many concats (10):

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