Better way to build mysqli prepared statement with some if [duplicate]

I have this portion of code in my underconstruction web app, there are 6 input var ($startDateEmissF,$endDateEmissFinto,$startDateScadF,$endDateScadFquery,$startDateEsecF,$endDateEsecF...they are all string)but they are used only within some conditions. Which is the better way to build a prepared statement with mysqli in this case where bind_param has different params depending on which If conditions are true or false?

Thank you

 $sql = "SELECT 
                    tblDocAdd.id_doc_add AS idDocAdd,
                    tblAz.ragSoc_azienda AS ragSoc,
                    tblScadAdd.id_scad_add AS idScadAdd,
                    tblAz.soprannome_azienda AS soprannomeAzienda,
                    tblCatDoc.categoriaDoc AS categoriaDoc,
                    tblMod1.nomeModalita AS nomeModalitaDoc,
                    tblDocAdd.nr_doc_add AS nrDoc,
                    tblDocAdd.descrizione_doc AS descrDocAdd,
                    tblScadAdd.importo_scad_add AS importoScadAdd,
                    tblScadAdd.importo_comp AS importoComp,
                    tblMod2.nomeModalita AS nomeModalitaScad,
                    tblScadAdd.data_scad_add AS dataScadAdd,
                    tblScadAdd.data_esecuzione AS dataEsecAdd,
                    tblScadAdd.note AS noteScadAdd,
                    tblConto.contoCorrente AS contoAdd,
                    tblDocAdd.data_emissione AS dataEmisDoc,
                    tblTipoAdd.tipologiaAddebito AS tipoAdd,
                    tblDocAdd.doc_approvato AS docApprovato
                    FROM $tabellaScadAdd AS tblScadAdd
                    INNER JOIN $tabellaDocAdd AS tblDocAdd ON tblScadAdd.id_doc_add = tblDocAdd.id_doc_add 
                    INNER JOIN $tabellaAziende AS tblAz ON tblDocAdd.id_azienda = tblAz.id
                    INNER JOIN $tabellaCatDoc AS tblCatDoc ON tblDocAdd.id_categoria = tblCatDoc.id_categoriaDoc
                    INNER JOIN $tabellaModalita AS tblMod1 ON tblDocAdd.id_modalita_doc = tblMod1.id_nomeModalita
                    INNER JOIN $tabellaModalita AS tblMod2 ON tblScadAdd.id_modalita_scad = tblMod2.id_nomeModalita 
                    INNER JOIN $tabellaTipoAdd AS tblTipoAdd ON tblDocAdd.id_tipologia = tblTipoAdd.id_tipologiaAddebito
                    INNER JOIN $tabellaContiCorr AS tblConto ON tblDocAdd.id_contoCorrente = tblConto.id_contoCorrente WHERE";


    $flag = 0;
    if ( isset( $_GET[ 'nonapprovati' ] ) ) {
      $sql .= " tblDocAdd.doc_approvato = 0 ";
      $flag = 1;
    }

    if ( !isset( $_GET[ 'nonapprovati' ] ) ) {
      $sql .= " tblDocAdd.doc_approvato = -1 ";
      $flag = 1;
    }

    if ( isset( $_GET[ 'eseguiti' ] ) ) {
      if ( $flag == 1 ) {
        $sql .= " AND ";
      }
      $sql .= " tblScadAdd.data_esecuzione IS NOT NULL ";
      $flag = 1;
    }

    if ( !isset( $_GET[ 'eseguiti' ] ) ) {
      if ( $flag == 1 ) {
        $sql .= " AND ";
      }
      $sql .= " tblScadAdd.data_esecuzione IS NULL ";
      $flag = 1;
    }

    if ( !empty( $startDateEmissF ) && !empty( $endDateEmissF ) ) {
      if ( $flag == 1 ) {
        $sql .= " AND ";
      }
      $sql .= " dataEmisDoc >= '$startDateEmissF' AND dataEmisDoc >= '$endDateEmissF' ";
      $flag = 1;
    }
    if ( !empty( $startDateScadF ) && !empty( $endDateScadF ) ) {
      if ( $flag == 1 ) {
        $sql .= " AND ";
      }
      $sql .= " dataScadAdd >= '$startDateScadF' AND dataScadAdd >= '$endDateScadF' ";
      $flag = 1;
    }
    if ( isset( $_GET[ 'eseguiti' ] ) && !isset( $_GET[ 'nonapprovati' ] ) ) {
      if ( !empty( $startDateEsecF ) && !empty( $endDateEsecF ) ) {
        if ( $flag == 1 ) {
          $sql .= " AND ";
        }
        $sql .= " dataEsecAdd >= '$startDateEsecF' AND dataEsecAdd >= '$endDateEsecF' ";
        $flag = 1;
      }
    }