Question

I have a string which contains a math formula, like T + ST + s + t …

I’m replacing all those letter identifiers with numbers using:

$ids = array(
    'T'    => $t1,
    'ST',  => $st,
    's',   => $s1,
    't',   => $t2,
    'N',   => 1,     
);

foreach ($ids as $id => $value) {
    if (strpos($formula, $id) !== false) {
        $formula = str_replace($id, $value, $formula);
    }
}

Which is ok in certain situations.
But if the formula has ST at the beginning I get a string like S345324 ..

I fixed this by moving ST in the first position in my array, but I feel it’s not really the best option 🙂

Are there any other “nicer” solutions?

Answer 1

Are you looking for strtr()?

$ids = array(
  'T'    => $t1,
  'ST'   => $st,
  's'    => $s1,
  't'    => $t2,
  'N'    => 1,     
);

$formula = strtr($formula, $ids);

Note that since strtr() always tries to find the longest possible match, it won’t replace occurrences of ST with S$t1 (instead of $st), regardless of how your $replace_pairs array is ordered.

---

Example (as seen on codepad):

$ids = array(
  'T'    => 10,
  'ST'   => 20,
  's'    => 30,
  't'    => 40,
  'N'    => 1,     
);

$formula = 'T + ST + s + t';
echo strtr($formula, $ids);

Prints:

10 + 20 + 30 + 40