A PHP trait to write data from WP-CLI to a CSV, in a memory-friendly and won't-lose-data-if-CLI-errors way.

trait-csv-report.php

<?php
/**
 * Simplify progressive generation of a CSV output file. Permits a command to
 * write each row to a CSV as records are considered, avoiding the need to
 * maintain the entire dataset in memory.
 */

/* phpcs:disable HM.Files.ClassFileName.MismatchedName */
// (HM's standards want this to be class-*, which is misleading.)

namespace MyProject\CLI;

trait CSV_Report {

	/**
	 * Dictionary of CSV resources, keyed by file path.
	 *
	 * @var array
	 */
	private $csv_report_handles;

	/**
	 * Create and store a file resource at a particular path.
	 *
	 * @param string $path Path of CSV to create.
	 */
	private function csv_report_open( string $path ) : void {
		if ( ! isset( $this->csv_report_handles ) ) {
			$this->csv_report_handles = [];
		}

		$this->csv_report_handles[ $path ] = fopen( $path, 'w' );
	}

	/**
	 * Append a row of data to the CSV at a specific path. If the CSV resouve
	 * does not yet exist, a file handler will be opened and the array keys
	 * of the received data object will be written as the first row in the file.
	 *
	 * @param string $path The path of the CSV file to write to.
	 * @param array  $data Associative array of data to write to the CSV file.
	 */
	protected function csv_report_append( string $path, array $data ) : void {
		if ( ! isset( $this->csv_report_handles[ $path ] ) ) {
			$this->csv_report_open( $path );

			fputcsv( $this->csv_report_handles[ $path ], array_keys( $data ) );
		}

		fputcsv( $this->csv_report_handles[ $path ], $data );
	}

	/**
	 * Create and store a file resource at a particular path.
	 *
	 * @param string $path Path of CSV to create.
	 */
	protected function csv_report_close( string $path ) : void {
		if ( isset( $this->csv_report_handles[ $path ] ) ) {
			fclose( $this->csv_report_handles[ $path ] );
		}
	}

	/**
	 * Wrap a bulk_task callback with a handler which will write the return
	 * value of each callback execution to a specified CSV. Return "false"
	 * from the passed callable to omit the result from the output file.
	 *
	 * @param callable $callback Alley_CLI_Bulk_Task bulk_task callback.
	 * @param string   $path     The path of the CSV file to write to.
	 * @return callable Wrapped callback.
	 */
	protected function csv_report_from_callback( callable $callback, string $path = '' ) : callable {
		if ( empty( $path ) ) {
			return $callback;
		}

		return function( $post ) use ( $callback, $path ) {
			$result = $callback( $post );

			if ( $result ) {
				$this->csv_report_append( $path, $result );
			}

			return $result;
		};
	}
}