tack is the correct engine implementation for any templating engine or substitution language.

• v1.1 - fix leaks, and misc. cleanup
• v1.0 - allocate exact output buffer size in constant time
• v0.2 - improved memory reallocation, option to keep unparsed variables
• v0.1 - proof of concept

CCBYSA 3.0 http://creativecommons.org/licenses/by-sa/3.0/

since this is a php project, the explanation will stick to php terminology.

in short, repeated calls to str_replace, or the use of arrays for search and replace variables, suffers from what is noted as the "replacement order gotcha" in the php documentation.

suppose you have the text body "cat", and you are to make two substitutions:

b => m
c => b

with str_replace your output is subject to implementation detail because the order in which these substitutions are run on the text body will affect the outcome. if the order is known then the algorithm can be taken advantage of to do clever things, and all Real Programmers understand that this is Good -- but it is not desired here.

what is actually desired of a templating engine is behavior identical to conway's game of life, where all cell states are resolved before any are changed. the output string in our example should always be "bat", because the usual point of templating involves producing markup which may clash with the templating language itself. ordered replacement may result in an intermediate state of the output which appears to contain instructions to perform a substition which was not called for.

in our simple example, "bat" is actually an intermediate state when substitutions are made last-to-first, and when the call returns the output is "mat". in practice, this will never be so benign; murphy's law dictates that the output is a damaging code-generated script that erases your hard disk, or hideous mangled html that gets you fired from your job as a guy that doesn't create mangled html.

if you are here trying to find a better templating engine because templating languages are too close to the domain language of the outputs you are dealing with, or because you tried to substitute in the contents of a template itself, you know exactly what i'm talking about.

this is relatively old code, and only contains project management files sufficient for php extensions at the time. good luck using it, i have no idea if it can still be compiled as such.

this project was conceived and created in january 2006. i am sure the authors of other php templating systems have figured out by now that they had this problem, and have corrected it, but at the time nobody had yet done so.