Example snippet to determine supported PHP versions using PHPCompatibility sniffs and phpcs.

phpcompat.php

<?php


function check_compatibility( $audit_files_directory, $phpcs_report_file, $phpcompat_report_file, $lowest_version = '5.2' ) {

	// Official PHP versions.
	// Note: Does not include patch releases as PHPCompatibility doesn't get this granular.
	$php_versions = array(
		'5.2',
		'5.3',
		'5.4',
		'5.5',
		'5.6',
		'7.0',
		'7.1',
		'7.2',
	);

	$compatible_versions   = array(); // PHP versions that this code is compatible with.
	$compat = array(); // Sniff results keyed by PHP version.

	// Run phpcs with PHPCompatibility standard.
	// Note: The $lowest_version is important as it will flag all errors from most recent version to the $lowest_version.
	exec( 'phpcs -p ' . $audit_files_directory . ' --standard=PHPCompatibility --runtime-set testVersion ' . $lowest_version . ' --report=json --report-file=' . $phpcs_report_file );

	// Only proceed if phpcs successfully created a report file.
	if ( file_exists( $phpcs_report_file ) ) {

		$json = file_get_contents( $phpcs_report_file );
		$json = json_decode( $json, true );

		// Map errors into an array keyed by PHP version.
		foreach ( $json['files'] as $filename => $errors ) {
			foreach ( $errors['messages'] as $message ) {
				$version = [];
				preg_match( '/(\d)(.\d)+/', $message['message'], $version );
				if ( ! array_key_exists( $version['0'], $compat ) ) {
					$compat[ $version['0'] ] = array( 'files' => array() );
				}
				$compat[ $version['0'] ]['files'][ $filename ]['messages'][] = $message;
			}
		}

		ksort( $compat );
		// Output the new keyed results as json file.
		file_put_contents( $phpcompat_report_file, json_encode( $compat ) );

		// Get all versions that report errors...
		$versions = array_keys( $compat );
		// ... sort them from higest to lowest...
		asort( $versions );
		// ... get the highest version ...
		$highest_version     = array_pop( $versions );
		// ... then slice the array to only contain remaining compatible versions.
		$offset              = array_search( $highest_version, $php_versions );
		$compatible_versions = array_slice( $php_versions, $offset + 1 );
	}

	// Return the results.
	return array(
		'compatible_versions' => $compatible_versions,
		'details' => $compat,
	);
}

// Example:
check_compatibility( '.', './report.json', './compatibility.json' );