how to convert a character to 7 bit even parity in php

I want to convert a Character to a 7 bit even parity. Can you please suggest me, how to implement this?

Too bad you can't use the x86 JPO instruction (Jump if Parity Odd) ;-)

Depending on the amount of data you want to handle it might be faster if you first set up a translation table than to check/handle character by character.

$map = array();
for($char=0; $char<128; $char++) {
  $parity = 0;
  for($bit=0; $bit<8; $bit++) {
    if($char & (1<<$bit)) {
      $parity ^= 128;
    }
  }
  $map[chr($char)] = chr($char|$parity);
}

(you might want to test this code thoroughly, I haven't)
and then use strtr() to translate from ascii7 to ascii7-evenbit.

$input = 'mary had a little lamb'; // all characters must be within the ascii7 range
$evenbit = strtr($input, $map);
// test output
for($i=0; $i<strlen($evenbit); $i++) {
  printf("%08s\n", decbin(ord($evenbit[$i])));
}