Changing array values in PHP
Let me first review this from an readability point of view, then answer your question and an proposed solution.

Naming

Your variable names could be more descriptive.


$array -> (maybe) $conjugations (german: $konjugationen)
$stamm -> $word_stem (german: $wort_stamm)
$zeiten -> $tenses (german: $zeitformen)?


Structure

The most intriguing part for me is $array's structure.
As I understand it, it is a 2D table of conjugations of the $verb where rows have the same tense and columns the same person. This should be somehow reflected in the data structure because it is not obvious to the viewer.
Maybe you should even implement a class for that.

Comparison

Maybe it has to do with my lack of domain knowledge (I don't speak french) but I find both solutions unreadable.
This is enforced by the naming problems.

The second one looks slightly more general with regard to reuse but the if portion is not very flexible.

Improvement

As I said, I have no idea about french but I suppose this holds true:

Each verb can be conjugated for person and tense. There is a set of rules that defines how this happens for each person and tense depending on the verb's form and a set of exemptions from those rules.

The solution to the problem consists of:


Classifying the verb as to which rule/exemption to apply
applying the rule


To me this sounds like 1. could be solved by a factory that creates strategy instances for 2.

$conjugator = get_conjugator($verb, $tense, $person);
$conjugated_verb = $conjugator-&gt;conjugate($verb);


Maybe the exceptions/rules are even partial per tense or person. This would mean you would have to make this distinction as well and produce a separate conjugator for each tense, person, verb triple.

Now the code you have posted is pretty much the part that goes into the implementation of a conjugator. And seeing that there could be many of them it would be nice to make implementing them as simple as possible.

Their interface would probably consist of a function that detects whether they are applicable (the if condition in your case) and a function that does the actual flexion of the input (the if's then code block).