PHP find string knowing only beginning of it

This sounds like a good match for a regex:

$regex = "/\'Dude=([0-9]+)\'/";
$str = "'Duck=123456' 'Chicken=982731' 'Dude=123487' 'Boat=129832'";

preg_match_all($regex, $str, $matches, PREG_SET_ORDER, 0);

echo $matches[0][1]; // prints: 123487

If your input string/text has line-breaks, you will probably need additional flags for the regex matcher. But there are already questions and answers for this available, so please refer to the search.

You should use "str_replace()" function for this

echo str_replace("dude=","","dude=99999");

Here first parameter your fix string which would be start as above , second parameter is replacement string, which is put as empty , third parameter is your actual string

If you're going to check all lines for different strings you can do as I have done recently:

// loop through all uploaded files:
for ($key = 0; $key < count($_FILES['file']['tmp_name']); $key++) {

  // make an array of lines from a single file:
  $txt_file = file($_FILES['file']['tmp_name'][$key]);

  // loop through this array, line by line:
  foreach ($txt_file as $line) {

    // if your file is not utf-8 fix the encoding:
    $line = iconv("windows-1251", "utf-8", $line);

    // check if this string begins with 'Dude=':
    if (substr($line, 0, strlen('Dude=')) === 'Dude=') {

      // if yes, then trim the 'Dude=' and get what's after it:
      $found_value[$key]['dude'] = trim(substr($line, strlen('Dude=')));

      // if this string begins with 'Dude=' it cannot begin with something else,
      // so we can go straight to the next cycle of the foreach loop:
      continue;

      // check if this string begins with 'Something else=':
    } elseif (substr($line, 0, strlen('SomethingElse=')) === 'SomethingElse=') {

      // if yes, then trim the 'SomethingElse=' and get what's after if:
      $found_value[$key]['something_else'] = trim(substr($line, strlen('SomethingElse=')));

      // if there's more items to find in the string use 'continue' to shorten server's work
      continue;
    }
  }
}

I've adapted and commented it for you. Also you probably want to check if file(s) was uploaded properly, so you can use file_exists and is_uploaded_file

In the end you will get an array with structure like this:

$found_value [
  1 => array()[
    'dude' => 9999999
    'something_else' => 'hello'
  ]
  2 => array()[
    'dude' => 3765234
    'something_else' => 'hello again'
  ]
]