In languages like Java and C#, strings are immutable and it can be computationally expensive to build a string one character at a time. In said languages, there are library classes to reduce this cost such as C# System.Text.StringBuilder and Java java.lang.StringBuilder.
Does php (4 or 5; I'm interested in both) share this limitation? If so, are there similar solutions to the problem available?
No, there is no type of stringbuilder class in PHP, since strings are mutable.
That being said, there are different ways of building a string, depending on what you're doing.
echo, for example, will accept comma-separated tokens for output.
// This...
echo 'one', 'two';
// Is the same as this
echo 'one';
echo 'two';
What this means is that you can output a complex string without actually using concatenation, which would be slower
// This...
echo 'one', 'two';
// Is faster than this...
echo 'one' . 'two';
If you need to capture this output in a variable, you can do that with the output buffering functions.
Also, PHP's array performance is really good. If you want to do something like a comma-separated list of values, just use implode()
$values = array( 'one', 'two', 'three' );
$valueList = implode( ', ', $values );
Lastly, make sure you familiarize yourself with PHP's string type and it's different delimiters, and the implications of each.