Kelvin would like to do the following, show how you are in txt and also how you want the result, so that I can understand and help you better. A while ago I made a post about UTF-8 fix which parses a file in question and fixes and creates a new one, I'll leave the link here may be helpful. link

I made a code that verifies the txt generates a vector with all the numbers and their names together of a new vector with all the information, after that I run again the vector already correcting the field that is missing, with that correcting the names. and finally create the items separated by ";"

Watch out for the code:

<?php

$file_name = "IMP.txt";

$file = fopen($file_name, "r") or die ("Arquivo não encontrado!");
$file_output = fopen("Output_".$file_name, "w") or die ("Arquivo não criado!");
// Gerando o vetor para tratamento
while(!feof($file)){    
    $strAnalise =  fgets($file) ;
    $strAnaliseArray = explode(";", $strAnalise);
    if(trim($strAnaliseArray[7]) != ''){
        $ArrayChaveNome[$strAnaliseArray[1]] = trim($strAnaliseArray[7]);
    }
    $strArrayDados[]=$strAnaliseArray;
}
// Corrindo o vetor;
foreach ($strArrayDados as $key => $value) {
    if(trim($value[7]) == ''){
        if(array_key_exists($value[1], $ArrayChaveNome)){
            $strArrayDados[$key][7]= $ArrayChaveNome[$value[1]];
        }
    }
}
// retornando a ";"
foreach ($strArrayDados as $key => $value) {
    $resultPontoVirgula.= implode(";", $value);
    $resultPontoVirgulaToTxt= implode(";", $value);
    fwrite($file_output, $resultPontoVirgulaToTxt);
}   
// var_dump($strArrayDados);
// var_dump($strAnaliseArray);
var_dump($resultPontoVirgula);

fclose($file_output);
fclose($file);

?>

Test script

/*
Esse array simula a relação entre os códigos dos pedidos e os nomes
*/
$orders = array(
    '205537' => 'FULANO 1',
    '203885' => 'FULANO 2'
);

/*
Dados para teste
*/
$data = '100400;205537;30141;BALCAO 1 PORTA PER 400X670X500;MDF BP BRANCO;6;79281905;NOME;0205537006
100400;203885;50058;PAI OPC EDIT LARGX ALTX 25;BLANCHE#695#320;18;79283198;NOME;0203885018
100400;203885;100001;EMBALAGEM DE FERRAGENS DIVERSAS;                                ;35;79283215;                              ;0203885035
100400;203885;101619;EMBALAGEM DE FUNDOS DORMITORIO;                                ;36;79283216;                              ;0203885036';

/*
Essa é a parte que interessa.
*/
$arr = explode("\n", $data);
foreach ($arr as $k => $v) {
    $csv[$k] = str_getcsv($v, ';');
    $csv[$k][7] = $orders[$csv[$k][1]];
    $csv[$k] = implode(';', $csv[$k]);
}
$data = implode("\n", $csv);
echo $data;

Final implementation

$orders = array(
    '205537' => 'FULANO 1',
    '203885' => 'FULANO 2'
);

$file = '/local/do/arquivo.txt';
$arr = file($file);
foreach ($arr as $k => $v) {
    $csv[$k] = str_getcsv($v, ';');
    $csv[$k][7] = $orders[$csv[$k][1]];
    $csv[$k] = implode(';', $csv[$k]);
}
file_put_contents($file, (implode("\n", $csv));

Please be aware that this is not optimized for heavy files over 100MB, for example.