<?
$str = <<<EOT
FooID: 123456
Name: Chuck
When: 01/02/2013 01:23:45
InternalID: 
User Message: Hello,
this is nillable, but can be quite long. Text can be spread out over many lines
This\: works too. And can start with any number of \\n's. It can be empty, too.
What's worse, though is that this CAN contain colons (but they're _"escaped"_
 
 
using `\`) like so `\:`, and even basic markup!
EOT;
 
$arr = explode("\n", $str);
 
$prevKey = '';
$split = ': ';
$output = array();
for ($i = 0, $arrlen = sizeof($arr); $i < $arrlen; $i++) {
  $keyValuePair = explode($split, $arr[$i], 2);
  // ?: Is this a valid key/value pair
  if (sizeof($keyValuePair) < 2 && $i > 0) {
    // -> Nope, append the value to the previous key's value
    $output[$prevKey] .= "\n" . $keyValuePair[0];
  }
  else {
    // -> Maybe
    // ?: Did we miss an escaped colon
    if (substr($keyValuePair[0], -1) === '\\') {
      // -> Yep, this means this is a value, not a key/value pair append both key and
      // value (including the split between) to the previous key's value ignoring
      // any colons in the rest of the string (allowing dates to pass through)
      $output[$prevKey] .= "\n" . $keyValuePair[0] . $split . $keyValuePair[1];
    }
    else {
      // -> Nope, create a new key with a value
      $output[$keyValuePair[0]] = $keyValuePair[1];
      $prevKey = $keyValuePair[0];
    }
  }
}
 
var_dump($output);

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