Philip created this unit by lengthening the spear to the point of becoming a true pike, removing the heavy armor, and replacing the large shield with a smaller alternative.
The name "foot companions" was coined by Alexander III during the reign of his father Philip II as a recruiting method.
The length of the sarissa, while making them terrifying for an enemy to oppose, severely limited their maneuverability; and if they were taken in flank or rear they had little chance of responding.
This was particularly clear at the Battle of Gaugamela in 331 BC, when the rapid advance of the right wing caused a breach to open between two of the battalions of pezhetairoi—a force of enemy cavalry broke through and, had it not been for a lack of discipline in their own command, under Parmenion, and for Alexander's placing of a second line of traditional hoplites in reserve, the phalanx might have been destroyed from the rear.
[1] Apart from in pitched battles, the pezhetairoi and their sarissas were not very practical; it is supposed that they were re-armed, and their tactics adapted, to suit the guerrilla warfare that was prevalent, and necessary, in Bactria and Sogdia.