parse_config.php

<?php

$configString = 'opcja1 wartość;
opcja2 wartość;
blok1 {
  opcja3 wartość;
  opcja4 wartość;

  blok1.1 {
    opcja5 wartość;
  }
}

blok2 {
  opcjax 11;
}';

function parse_config_block( $input, &$n = 0 )
{
    $output = array();

    $currentString = '';

    for ( ; $n < strlen( $input ); $n++ )
    {
        $char = $input[ $n ];

        if ( $char == "\n" )
        {
            $currentString = '';

            continue;
        }

        if ( $char == ";" )
        {
            $currentString = trim( $currentString );

            if ( empty( $currentString ) )
            {
                throw new Exception('Invalid statement');
            }

            $statement = explode(' ', $currentString, 2);

            if ( count( $statement ) < 2 )
            {
                throw new Exception('Invalid statement');
            }

            $key = array_shift( $statement );

            $output[ $key ] = array_shift($statement);

            $currentString = '';

            continue;
        }

        if ( $char == '{' )
        {
            if ( empty( $currentString ) )
            {
                throw new Exception('Invalid statement at line');
            }

            $blockName = trim( $currentString );

            $n++;
            $output[ $blockName ] = parse_config_block( $input, $n );

            continue;
        }

        if ( $char == '}' )
        {
            return $output;
        }

        $currentString .= $char;
    }

    return $output;
}

var_dump( parse_config_block( $configString ) );