silverstripe-environmentcheck/code/checks/HasFunctionCheck.php

<?php

/**
 * Check that the given function exists.
 */
class HasFunctionCheck implements EnvironmentCheck {
	/**
	 * @var string
	 */
	protected $functionName;

	/**
	 * @param string $functionName The name of the function to look for.
	 */
	function __construct($functionName) {
		$this->functionName = $functionName;
	}

	/**
	 * @inheritdoc
	 *
	 * @return array
	 */
	function check() {
		if(function_exists($this->functionName)) return array(EnvironmentCheck::OK, $this->functionName.'() exists');
		else return array(EnvironmentCheck::ERROR, $this->functionName.'() doesn\'t exist');
	}
}