How to expand variables in a string - php

Question

Problem
I'd like to expand variables in a string in the same manner that variable in a double quoted string get expanded.
$string = '<p>It took $replace s</>';
$replace = 40;
expression_i_look_for;
$string should become '<p>It took 40 s</>';
I see a obvious solution like this:
$string = str_replace('"', '\"', $string);
eval('$string = "$string";');
But I really don't like it, because eval() is insecure. Is there any other way to do this ?
Context
I'm building a simple templateing engine, that's where I need this.
Example Template (view_file.php)
<h1>$title</h1>
<p>$content</p>
Template rendering (simplified code):
$params = array('title' => ...);
function render($view_file, $params)
extract($params)
ob_start();
include($view_file);
$text = ob_get_contents();
ob_end_clean();
expression_i_look_for; // this will expand the variables in the template
return $text;
}
The expansion of the variables in the template simplifies it's syntax. Without it, the above example template would be:
<h1><?php echo $title;?></h1>
<p><?php echo $content;?></p>
Do you think this approach is good ? Or should I look in another direction ?
Edit
Finally I understand that there is no simple solution due to flexible way PHP expands variables (even ${$var}->member[0] would be valid.
So there are only two options:
Adopt an existing full fledged templating system
Stick with something very basic that essentially is limited to including the view files via include.

Answer 1

I would rather suggest using some existing template engines, like for example Smarty, but if you really want to do it by yourself you can use the simple regular expression to match all variables constructed with for example letters and numbers and then replace them with correct variables:
<?php
$text = 'hello $world, what is the $matter? I like $world!';
preg_match_all('/\$([a-zA-Z0-9]+)/',
$text,
$out, PREG_PATTERN_ORDER);
$world = 'World';
$matter = 'matter';
foreach(array_unique($out[1]) as $variable){
$text=str_replace('$'.$variable, $$variable, $text);
}
echo $text;
?>
prints
hello World, what is the matter? I like World!

Answer 2

Parse
Parse the string look for $ followed by valid variable name (i.e. \[a-zA-Z_\x7f-\xff\]\[a-zA-Z0-9_\x7f-\xff\]*)
Variable²
Use variable variables syntax (i.e. $$var notation).

Answer 3

Are you trying to do this?
templater.php:
<?php
$first = "first";
$second = "second";
$third = "third";
include('template.php');
template.php:
<?php
echo 'The '.$first.', '.$second.', and '.$third.' variables in a string!';
When templater.php is run, produces:
"The first, second, and third variables in a string!"

Answer 4

Do you want something like this ?
$replace = 40;
$string = '<p>It took {$replace}s</p>';

Answer 5

Instead of using single quotes
$string = '<p>It took $replace s</>';
$replace = 40;
use double quotes
$replace = 40;
$string = "<p>It took $replace s</>";
However, for readability and to enable you to remove the space between $replace and the s I would use:
$replace = 40;
string = '<p>It took ' . $replace . 's</>';

Answer 6

The correct way is probably to parse your document as a tree, identify your parser tags ( because you are managing your own parser they don't have to follow php conventions if you don't want them to ) and then add in your values from an associative array or other data structure as the opportunity arises.
This is a more complex solution but will make it far easier when you realise that you want to be able to display lists whose length is unknown ahead of time using some kind of looping structure based on a standard display option. In the long run, you won't find many serious templating systems that aren't parsing the documents into some kind of in-memory tree where the placeholders can be inserted and then the document constructed as required. This also offers many opportunities for cacheing. Also, if you are unafraid of recursion you will be able to perform a lot of operations on it fairly simply.
However, this is not an uncommon problem to solve and as I commented on the question, there are almost guaranteed to be libraries and extensions around that provide most of the functionality you need. Unless this is a purely academic process for you, I would find some existing solutions and either use one of those or get a solid understanding of how it works so you have a starting point for adapting your own solution.

Answer 7

This is a snippet I pulled out from Lejlot's answer. I tested it and it works fine.
function resolve_vars_in_str( $input )
{
preg_match_all('/\$([a-zA-Z0-9]+)/', $input, $out, PREG_PATTERN_ORDER);
foreach(array_unique($out[1]) as $variable) $input=str_replace('$'.$variable, $GLOBALS["$variable"], $input);
return $input ;
}