awk '
    BEGIN { unique_str = "unique thing"; }          # stick in the unique string here

    {
        partial = 0;
        while( match( $0, "<\\?php [^\\?]*\\?>" ) > 0 )         # complete beginning/ending on the line
        {
            if( index( substr( $0, RSTART, RLENGTH ), unique_str ) )        # if it contains the magic string
                printf( "%s", substr( $0, 1, RSTART-1 ) );      # print evrything before it, and skip it
            else
                printf( "%s", substr( $0, 1, RSTART + (RLENGTH-1) ) );      # print everything including the begin and end  (edited)

            $0 = substr( $0, RSTART + RLENGTH  );

            partial = 1;
        }

    }

    /<?php$/ { drop = idx = 0; snarf = 1; } # start of a block; start buffering

    /?>/ {                  # end of a block
        if( ! drop )        # magic string not found -- show this block
        {
            for( i = 0; i < idx; i++ )
                printf( "%s\n", buffer[i] );
            printf( "%s\n", $0 );
        }

        snarf = 0;          # turn off buffering
        next;
    }

    match( $0, unique_str ) { drop = 1; }   # magic string found, drop if were in a php block

    snarf {                 # if buffering, hold the record until end of block reached.
        buffer[idx++] = $0;
        if( partial )
            printf( "\n" );
        next;
    }

    { print; }              # not buffering, just print the record.

'

awk '
    BEGIN { unique_str = "unique thing"; }          # stick in the unique string here

    {
        while( match( $0, "<\\?php [^\\?]*\\?>" ) > 0 )     # complete beginning/ending on the line
        {
            if( index( substr( $0, RSTART, RLENGTH ), unique_str ) )        # if it contains the magic string
                printf( "%s", substr( $0, 1, RSTART - 1 ) );        # print evrything before it, and skip it
            else
                printf( "%s", substr( $0, 1, RSTART + (RLENGTH-1) ) );      # print everything including the begin and end

            $0 = substr( $0, RSTART + RLENGTH  );  
        }

    }

    /<?php$/ { drop = idx = 0; snarf = 1; } # start of a block; start buffering

    /?>/ {                  # end of a block
        if( ! drop )        # magic string not found -- show this block
        {
            for( i = 0; i < idx; i++ )
                printf( "%s\n", buffer[i] );
            printf( "%s\n", $0 );
        }
        else
        {
            if( (i = index( buffer[0], "<?php" )) > 0 )    # if something before <?php, and we dropped the block, print the leading text
                printf( "%s\n", substr( buffer[0], 1, i-1 ) );
        }

        snarf = 0;          # turn off buffering
        next;
    }

    match( $0, unique_str ) { drop = 1; }   # magic string found, drop if were in a php block

    snarf {                 # if buffering, hold the record until end of block reached.
        buffer[idx++] = $0;
        if( partial )
            printf( "\n" );
        next;
    }

    { print; }              # not buffering, just print the record.
'