Source

File: wp-includes/class-wp-theme.php

public function get( $header ) {
		if ( ! isset( $this->headers[ $header ] ) ) {
			return false;
		}

		if ( ! isset( $this->headers_sanitized ) ) {
			$this->headers_sanitized = $this->cache_get( 'headers' );
			if ( ! is_array( $this->headers_sanitized ) ) {
				$this->headers_sanitized = array();
			}
		}

		if ( isset( $this->headers_sanitized[ $header ] ) ) {
			return $this->headers_sanitized[ $header ];
		}

		// If themes are a persistent group, sanitize everything and cache it. One cache add is better than many cache sets.
		if ( self::$persistently_cache ) {
			foreach ( array_keys( $this->headers ) as $_header ) {
				$this->headers_sanitized[ $_header ] = $this->sanitize_header( $_header, $this->headers[ $_header ] );
			}
			$this->cache_add( 'headers', $this->headers_sanitized );
		} else {
			$this->headers_sanitized[ $header ] = $this->sanitize_header( $header, $this->headers[ $header ] );
		}

		return $this->headers_sanitized[ $header ];
	}