test a string to see if it's a valid CSS unit, either by itself or by removing the integers

test-css-string.php

<?php
function confirm_css_unit( $string ) {

	// First check for a null or allowed strings.
	if ( empty( $string ) || null === $string || in_array( $string, [ 'auto', 'fit-content', 'inherit' ] ) ) {
		return $string;
	}

	// Pull out the numerical portion.
	$value = preg_replace( '~\D~', '', $string );

	// If we have no numbers at all, return 'auto'
	// so hopefully it does not look weird.
	if ( empty( $value ) ) {
		return 'auto';
	}

	// If the result matches our string, then it is only a number and
	// we need to add the default px suffix and return it.
	if ( $value === $string ) {
		return $string . 'px';
	}

	// Determine what our suffix is.
	$suffix = str_replace( $value, '', $string );

	// If our suffix is one of the allowed ones, return the original string.
	if ( in_array( $suffix, [ 'px', '%', 'ch', 'em', 'rem', 'pt', 'in', 'cm', 'mm', 'ex', 'pc', 'vh', 'vw', 'vmin', 'vmax' ] ) ) {
		return $string;
	}

	// Since it wasn't one that we allow, return the numeric value with a px attached.
	return $value . 'px';
}