Export custom select field. Refer to https://github.com/phacility/phabricator/pull/845/commits/77b126b73fa18fc367cfb96b27f0b7163ef6f92f

\src\infrastructure\customfield\standard\PhabricatorStandardCustomFieldSelect.php

<?php

final class PhabricatorStandardCustomFieldSelect
  extends PhabricatorStandardCustomField {
  
  //***************//
  //Existing Code here
  //**************//
  
  /*
  * Adding new function
  */
  protected function newExportFieldType() {
    return new PhabricatorSelectExportField($this); //Need to inject a reference to the instance of the object
  }

}

\src\infrastructure\export\field\PhabricatorSelectExportField.php

<?php

final class PhabricatorSelectExportField
  extends PhabricatorExportField {

  private $phabricatorStandardCustomSelectField;

  function __construct($_phabricatorStandardCustomSelectField){
        $this->phabricatorStandardCustomSelectField = $_phabricatorStandardCustomSelectField;
  }

  

  public function setPhabricatorStandardCustomSelectField($value){
      $this->phabricatorStandardCustomSelectField = $value;
      return $this;
  }

  public function getPhabricatorStandardCustomSelectField(){
    return $this->phabricatorStandardCustomSelectField;
  }

  public function getTextValue($value) {
    $field =  $this->getPhabricatorStandardCustomSelectField();
    $field_Options = $field->getOptions();
    return $field_Options[$value];
  }
}