Example code for action that runs after Caldera Forms submission is processed, but before it is saved. For more information see: https://calderaforms.com/doc/caldera_forms_submit_post_process/

caldera_forms_submit_post_process-get-and-change.php

<?php
add_action( 'caldera_forms_submit_post_process', function( $form, $referrer, $process_id, $entry_id  ){
	//make sure to set your field ID here.
	$field_id = 'fld_123456';

	//Get value of a field
	$field_value = Caldera_Forms::get_field_data( $field_id, $entry_id, $form );

	//check value
	if( in_array( $field_value, array( 'Roy', 'Shawn' )) ){
		//change value
		Caldera_Forms::set_field_data( $field_id, 'Mike', $form, $entry_id );
	}

}, 10, 4 );

caldera_forms_submit_post_process-session.php

<?php
add_action( 'caldera_forms_submit_post_process', function( $form, $referrer, $process_id, $entry_id  ){
   //Save field data from a SESSION var if it is set
   if( isset( $_SESSION, $_SESSION[ 'foo' ] ) ){
      //make sure to update the field ID
      Caldera_Forms::set_field_data( 'fld1234567', $_SESSION[ 'foo' ], $form, $entry_id );
   }

}, 10, 4 );

caldera_forms_submit_post_process-use-post.php

<?php
add_action( 'caldera_forms_submit_post_process', function( $form, $referrer, $process_id, $entry_id ){

	//IMPORTANT -- Change the form ID to match your form
	if( 'CF57c872a1c80af' == $form[ 'ID' ] ){

		//IMPORTANT -- Change the field ID to match your post select field
		$field_id = 'fld_5702696';
		
		//get the Post ID from current field value
		$post_id = Caldera_Forms::get_field_data( $field_id, $form, $entry_id );
		if( is_numeric( $post_id ) ){
			
			//get title, by ID and set it as field value
			$title = get_the_title( $post_id );
			Caldera_Forms::set_field_data( $field_id, $title, $form, $entry_id );
		}
	}

}, 10, 4 );

@Shelob9 First example l7 sets the arguments in the wrong order. It should be $field_value = Caldera_Forms::get_field_data( $field_id, $form, $entry_id );