Very simple, quite powerful, extensible, plain-PHP one-class template engine.

Angie.php

<?php namespace GM;

/**
 * Angie. Very simple, and quite powerful, plain PHP template engine.
 *
 * @author  Giuseppe Mazzapica <giuseppe.mazzapica@gmail.com>
 * @license http://opensource.org/licenses/MIT MIT
 */
class Angie
{
    /**
     * @var array
     */
    private $data;

    /**
     * @var bool
     */
    private $strictVars;

    /**
     * @var bool|string
     */
    private $rendering = false;

    /**
     * @var array
     */
    private $functions = [];

    /**
     * @param array $data       Pre-defined data
     * @param bool  $strictVars
     */
    public function __construct(array $data = [], $strictVars = false)
    {
        $this->data = $data;
        $this->strictVars = $strictVars;
    }

    /**
     * Allow to call registered functions from template files.
     *
     * Registered functions can be called in template files using `$this->functionName()`
     * or the shorter alias `$A->functionName()`
     *
     * @param  string $name
     * @param  array  $args
     * @return mixed
     * @see Angie::registerFunction()
     */
    public function __call($name, array $args)
    {
        if ( ! $this->rendering) {
            throw new \LogicException(
                sprintf('%s() function only available when rendering.', $name)
            );
        }

        $exists = array_key_exists($name, $this->functions);

        if (! $exists && ! function_exists($name)) {
            throw new \RuntimeException(
                sprintf('%s is not a registered function.', $name)
            );
        }

        $callable = $exists ? $this->functions[$name] : $name;

        ob_start();
        $return = call_user_func_array($callable, $args);
        $output = ob_get_clean();

        return $output ? : $return;
    }

    /**
     * Allow to call template variables in template files.
     *
     * Variables not set are rendered as empty string (no need to check for isset()) unless the
     * `$strictVars` object var is set to `true` ins constructor.
     *
     * @param  string $name
     * @return string
     */
    public function __get($name)
    {
        if (! $this->rendering) {
            throw new \LogicException('Variable access is only available when rendering.');
        }

        $exists = array_key_exists($name, $this->data);

        if (! $exists && $this->strictVars) {
            throw new \RuntimeException(sprintf('Unresolved var: %s.', $name));
        }

        return $exists ? $this->data[$name] : '';
    }

    /**
     * Render a template with given data.
     *
     * Given data is merged with base data passed to constructor.
     *
     * Variable to be replaced with data have to be written using the syntax
     * (similar to `sprintf` syntax):
     *
     * ```
     * Hello <b>%user_name$s<b>,<br>
     * your last access was <span class="last-access">%last_access_days$s ago</span>.
     * ```
     *
     * Where the variable names (that must match `$data` keys) are:
     * `user_name` and `last_access_days`.
     *
     * This syntax is, of course, available only for string variables.
     *
     * This syntax allows to write template without the need to use PHP code,
     * e.g. just passing arbitrary string.
     * 
     * However, when using template files, they're regular PHP files that may contain any PHP code.
     *
     * To access variables in template files is possible to use `$this->var_name`
     * or the shorter alias `$A->var_name`.
     * 
     * In the same way is possible to call Angie methods (e.g. `partial()`)
     * and all the register functions.
     *
     * @param  string $template Arbitrary or full path to a template file
     * @param  array  $data
     * @return string
     */
    public function render($template = '', $data = [])
    {
        $baseData = $this->data;
        $this->data = array_merge($this->data, $data);

        $template = $this->maybeFile($template);

        // replace sprintf-like vars
        if (preg_match_all('~(?:^|[^\w])(%([a-zA-Z_]+)\$s)(?:\b)~m', $template, $matches)) {
            $search = array_unique($matches[1]);
            // muted to avoid double exception when `getReplace()` throws and exception
            $replace = @array_map([$this, 'getReplace'], array_unique($matches[2]));
            $template = str_replace($search, $replace, $template);
        }

        // reset data so additional calls to `render()` are not affected by template-specific data
        $this->data = $baseData;

        return $template;
    }

    /**
     * Inside template files this method can be used to render a partial template.
     *
     * Given data is merged with the "parent" template data if `$isolate` argument
     * is false-ish.
     *
     * The method can be called in template files using `$this->partial()`
     * or the shorter alias `$A->partial()`.
     *
     * @param  string $partial Full path to a template file
     * @param  array  $data
     * @param  bool   $isolate
     * @return string
     */
    public function partial($partial, array $data = [], $isolate = false)
    {
        if (! is_string($partial)) {
            throw new \InvalidArgumentException(
                sprintf('Partial path must be in a string.', $partial)
            );
        }

        if (! $this->rendering) {
            throw new \LogicException(
                sprintf('%s only available when rendering.', __METHOD__)
            );
        }

        $instance = clone $this;
        $instance->data = $isolate ? $data : array_merge($this->data, $data);

        // try to treat partial as relative path from the folder of the parent
        if (! is_file($partial)) {
            $relative = filter_var($partial, FILTER_SANITIZE_URL);
            $dir = dirname($this->rendering);
            $partial = $dir.DIRECTORY_SEPARATOR.ltrim($relative, '\\/.');
        }

        if (! is_file($partial)) {
            throw new \RuntimeException(
                sprintf('%s is not a valid partial template.', $partial)
            );
        }

        return $instance->render($partial);
    }

    /**
     * Using this method (only available outside template files) is possible
     * to register arbitrary callbacks that can then be used in template files,
     * and accessed with `$this->funcName()` or the shorter alias `$A->funcName()`.
     *
     * In this way is possible to write expressive and readable templates,
     * without having to write dozens of global PHP functions or dealing
     * with PHP objects and namespaces in template files.
     *
     * @param string   $name
     * @param callable $function
     */
    public function registerFunction($name, callable $function)
    {
        if ($this->rendering) {
            throw new \LogicException(
                sprintf('%s only available when not rendering.', __METHOD__)
            );
        }
        $this->functions[$name] = $function;
    }

    /**
     * If template is a file, loads it and return the output as template.
     * If template is an arbitrary string, just returns it.
     *
     * @param  string $template
     * @return string
     */
    private function maybeFile($template)
    {
        if (is_file($template) && is_readable($template)) {
            $this->rendering = $template;
            // inside template files the `$A` var is available to call Angie methods
            // like $A->partial()` or any of the callback registered,
            // using `registerFunction()` method
            $A = $this;
            ob_start();
            /** @noinspection PhpIncludeInspection */
            require $template;
            $template = ob_get_clean();
            $this->rendering = false;
        }

        return $template;
    }

    /**
     * Get the replacement value for variables wrote using sprintf-like syntax.
     *
     * @param  string $var
     * @return string
     */
    private function getReplace($var)
    {
        if ($this->strictVars && ! isset($this->data[$var])) {
            throw new \RuntimeException(sprintf('Var %s is not set.', $var));
        }

        $replace = isset($this->data[$var]) ? $this->data[$var] : '';

        if (is_scalar($replace)) {
            return (string) $replace;
        } elseif (is_object($replace) && method_exists($replace, '__toString')) {
            return $replace->__toString();
        }

        if ($this->strictVars) {
            throw new \InvalidArgumentException(sprintf('Var %s is not a string.', $var));
        }

        return '';
    }
}